51
arXiv:1409.3820v1 [math.NT] 6 Sep 2014 LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS AND APPLICATIONS (1878–2014) ROMEO ME ˇ STROVI ´ C ABSTRACT. In 1878 ´ E. Lucas proved a remarkable result which pro- vides a simple way to compute the binomial coefficient ( n m ) modulo a prime p in terms of the binomial coefficients of the base-p digits of n and m: If p is a prime, n = n 0 + n 1 p + ··· + n s p s and m = m 0 +m 1 p+··· +m s p s are the p-adic expansions of nonnegative integers n and m, then n m s i=0 n i m i (mod p). The above congruence, the so-called Lucas’ theorem (or Theorem of Lucas), plays an important role in Number Theory and Combinatorics. In this article, consisting of six sections, we provide a historical survey of Lucas type congruences, generalizations of Lucas’ theorem modulo prime powers, Lucas like theorems for some generalized binomial coef- ficients, and some their applications. In Section 1 we present the fundamental congruences modulo a prime including the famous Lucas’ theorem. In Section 2 we mention several known proofs and some consequences of Lucas’ theorem. In Section 3 we present a number of extensions and variations of Lucas’ theorem modulo prime powers. In Section 4 we consider the notions of the Lucas property and the double Lucas property, where we also present numerous integer sequences satisfying one of these properties or a certain Lucas type congruence. In Section 5 we collect several known Lucas type con- gruences for some generalized binomial coefficients. In particular, this concerns the Fibonomial coefficients, the Lucas u-nomial coefficients, the Gaussian q-nomial coefficients and their generalizations. Finally, some applications of Lucas’ theorem in Number Theory and Combina- torics are given in Section 6. 2010 Mathematics Subject Classification. Primary 11B75, 11A07, 05A10, 11B65; Sec- ondary 11B37, 11B39, 11B50, 11B73. Keywords and phrases: prime, prime power, binomial coefficient, congruence modulo a prime (prime power), p-adic expansion of an integer, Lucas’ theorem, Lucas’ congru- ence, Wolstenholme type congruence, Lucas type congruence, variation of Lucas’ theorem modulo prime powers, generalization of Lucas’ theorem, Lucas property, double Lucas property, generalized binomial coefficient, Fibonomial coefficient, Lucas u-nomial coeffi- cient, Gaussian q-nomial coefficient, Pascal’s triangle, p-Lucas property. 1

ROMEO MESTROVIˇ C´ arXiv:1409.3820v1 [math.NT] 6 Sep 2014 · m modulo a prime pin terms of the binomial coefficients of the base-pdigits of nand m: If pis a prime, n = n0 +n1p+···

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LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONSAND APPLICATIONS (1878–2014)

ROMEO MESTROVIC

ABSTRACT. In 1878E. Lucas proved a remarkable result which pro-vides a simple way to compute the binomial coefficient

(n

m

)modulo

a primep in terms of the binomial coefficients of the base-p digits ofn andm: If p is a prime,n = n0 + n1p + · · · + nsp

s andm =m0+m1p+· · ·+msp

s are thep-adic expansions of nonnegative integersn andm, then (

n

m

)≡

s∏

i=0

(ni

mi

)(mod p).

The above congruence, the so-calledLucas’ theorem(or Theorem ofLucas), plays an important role in Number Theory and Combinatorics.In this article, consisting of six sections, we provide a historical surveyof Lucas type congruences, generalizations of Lucas’ theorem moduloprime powers, Lucas like theorems for some generalized binomial coef-ficients, and some their applications.

In Section 1 we present the fundamental congruences modulo aprimeincluding the famous Lucas’ theorem. In Section 2 we mentionseveralknown proofs and some consequences of Lucas’ theorem. In Section3 we present a number of extensions and variations of Lucas’ theoremmodulo prime powers. In Section 4 we consider the notions of the Lucasproperty and the double Lucas property, where we also present numerousinteger sequences satisfying one of these properties or a certain Lucastype congruence. In Section 5 we collect several known Lucastype con-gruences for some generalized binomial coefficients. In particular, thisconcerns the Fibonomial coefficients, the Lucasu-nomial coefficients,the Gaussianq-nomial coefficients and their generalizations. Finally,some applications of Lucas’ theorem in Number Theory and Combina-torics are given in Section 6.

2010Mathematics Subject Classification.Primary 11B75, 11A07, 05A10, 11B65; Sec-ondary 11B37, 11B39, 11B50, 11B73.

Keywords and phrases: prime, prime power, binomial coefficient, congruence moduloa prime (prime power),p-adic expansion of an integer, Lucas’ theorem, Lucas’ congru-ence, Wolstenholme type congruence, Lucas type congruence, variation of Lucas’ theoremmodulo prime powers, generalization of Lucas’ theorem, Lucas property, double Lucasproperty, generalized binomial coefficient, Fibonomial coefficient, Lucasu-nomial coeffi-cient, Gaussianq-nomial coefficient, Pascal’s triangle,p-Lucas property.

1

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 2

CONTENTS

1 Introduction 3

2 Lucas’ theorem and its variations 5

2.1 Lucas’ theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .52.2 Some consequences and extensions of Lucas’ theorem . . . .. . . . . .7

3 Lucas type congruences for prime powers 10

3.1 Wolstenholme type congruences . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .103.2 Variations of Lucas’ theorem modulo prime powers . . . . . .. . . . . .113.3 Characterizations of Wolstenholme primes . . . . . . . . . . .. . . . . . . . .17

4 The Lucas property and thep-Lucas property 18

4.1 The Lucas property and the double Lucas property . . . . . . .. . . . . .184.2 Further Lucas type congruences . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .23

5 Lucas type theorems for some generalized binomial coefficients 27

5.1 Generalized binomial coefficients and related Lucas type congru-ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .27

5.2 Lucas type congruences for some classes of Lucasu-nomial coeffi-cients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .32

6 Some applications of Lucas’ theorem 36

6.1 Lucas’ theorem and the Pascal’s triangle . . . . . . . . . . . . .. . . . . . . . .366.2 Another applications of Lucas’s theorem . . . . . . . . . . . . .. . . . . . . . .40

References 43

Appendix 49

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 3

1. INTRODUCTION

Prime numbers have been studied since the earliest days of mathematics.Congruences modulo primes have been widely investigated since the timeof Fermat. There are numerous useful and often remarkable congruencesand divisibility results for binomial coefficients; see [36, Ch. XI] for olderresults and [52] for a modern perspective.

Let p be a prime. Then byFermat little theorem, for each integera notdivisible byp

ap−1 ≡ 1 (mod p).

Furthermore, byWilson theorem, for any primep

(p− 1)! + 1 ≡ 0 (mod p).

In attempting to discover some analogous expression which should bedivisible byn2, whenevern is a prime, but not divisible ifn is a compositenumber, in 1819 Charles Babbage [9] is led to the congruence

(2p− 1

p− 1

)≡ 1 (mod p2)

for all primesp ≥ 3. In 1862 J. Wolstenholme [142] proved that the abovecongruence holds modulop3 for any primep ≥ 5.

The study of arithmetic properties of binomial coefficientshas a rich his-tory. As noticed in [52], many great mathematicians of the nineteenth cen-tury considered problems involving binomial coefficients modulo a primepower (for instance Babbage [9], Cauchy, Cayley, Gauss [45], Hensel, Her-mite [57], Kummer [80], Legendre, Lucas [86] and [87], and Stickelberger).They discovered a variety of elegant and surprising theorems which are of-ten easy to prove. For more information on these classical results, theirextensions, and new results about this subject, see books ofDickson [36,Chapter IX] and Guy [53], while a more modern treatment of thesubject isgiven by A. Granville [52].

Suppose that a primep and pair of integersn ≥ m ≥ 0 are given. Abeautifultheorem of E. Kummerof 1852 ([80, pp. 115–116]; also see [36,p. 270]) states thatthe exact power of the primep which divides

(nm

)is

given by the number of “carries” whenm andn −m are added in baseparithmetic. This is a fundamental result in the study of divisibility propertiesof binomial coefficients.

If n = n0+n1p+· · ·+nsps andm = m0+m1p+· · ·+msp

s are thep-adicexpansionsof nonnegative integersn andm (so that0 ≤ mi, ni ≤ p − 1for eachi), then byLucas’s theoremestablished byEdouard Lucas in 1878

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 4

[86] (also see [36, p. 271] and [52]),(n

m

)≡

s∏

i=0

(ni

mi

)(mod p).

The same result is without proof also presented by Lucas in 1878, in SectionXXI of his massive journal paper [87, pp. 229–230].

This remarkable result by Lucas provides a simple way to compute thebinomial coefficient

(nm

)modulo a primep in terms of the binomial coef-

ficients of the base-p digits of n andm. The above congruence, the so-called Lucas’ theorem (or Theorem of Lucas) is a very important congru-ence in Combinatorial Number Theory and Combinatorics. In particular,this concerns the divisibility of binomial coefficients by primes. In this arti-cle, consisting of six sections, we provide a historical survey of Lucas typecongruences, generalizations of Lucas’ theorem modulo prime powers andLucas like theorems for some classes of generalized binomial coefficients.Furthermore, we present some known applications of Lucas’ theorem andcertain of its variations in Number Theory and Combinatorics.

This article is organized as follows. In Section 2 we mentionseveralknown algebraic and combinatorial proofs of Lucas’ theorem. We also givesome consequences and variations of Lucas’ theorem. In Section 3 wepresent a number of extensions and variations of Lucas’ theorem moduloprime powers. In Section 4 we consider the notions of the Lucas propertyand the double Lucas property. In this section we also present numerousinteger sequences satisfying one of these properties or a certain similar Lu-cas type congruence. In particular, these properties are closely related tothe divisibility properties of certain binomial coefficients, matrices, differ-ent binomial sums, Apery numbers, Delannoy numbers, Stirling numbers ofthe first and second kind etc. In Section 5 we collect several known Lucastype congruences for some generalized binomial coefficients. In particular,this concerns the Fibonomial coefficients, the Lucasu-nomial coefficients,the Gaussianq-nomial coefficients and some their generalizations. Finally,applications of Lucas’ theorem are given in Section 6 of thissurvey article.Some of these applications are closely related to the determination of num-ber of entries of Pascal’s triangle with a prescribed divisibility property. Wealso present some known primality criteria whose proofs arebased on Lu-cas’ theorem. Furthermore, we give certain known results concerning thecharacterizations of the algebraicity of some classes of formal power seriesin terms of the notion of thep-Lucas property.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 5

2. LUCAS’ THEOREM AND ITS VARIATIONS

2.1. Lucas’ theorem. As noticed above, ifn = n0 +n1p+ · · ·+nsps and

m = m0 +m1p + · · · +msps are thep-adic expansionsof integersn and

m such that0 ≤ mi, ni ≤ p − 1 for eachi = 0, 1, . . . , s, then a beautifulLucas’s theorem([86]; also see [52] ([86] and [36, p. 271]) states that

(1)

(n

m

)≡

s∏

i=0

(ni

mi

)(mod p).

(with the usual convention that(00

)= 1, and

(lr

)= 0 if l < r). The con-

gruence (1) was established by Lucas by considering patterns in Pascal’striangle. Furthermore, (1) is equivalent to the following Lucas’ earlier gen-eralization [86, p. 52] of an 1869 result of H. Anton [7, pp. 303–306] (alsosee [36, p. 271]):

(2)

(n

m

)≡(n div p

m div p

)(n mod p

m mod p

)(mod p),

wheren div p denotes the integer quotient ofn by a primep, andn mod pits remainder. The congruence (2) is in fact the equivalent form of Lucas’theorem which is often stated in the follwing way:

(3)

(np+ r

mp+ s

)≡(n

m

)(r

s

)(mod p),

wherep is a prime,n,m, r ands are nonnegative integers such that0 ≤r, s ≤ p− 1.

If a primep divides(nm

)then (1) follows easily from Kummer’s theorem.

However, ifpl is the exact power ofp dividing(nm

), then we might ask for

the value of1pl

(nm

)(mod p). The related result was discovered by H. Anton

in 1869 [7] (see also [52], [75, pp. 3–4] and [121]) who provedthat if pl isthe exact power ofp dividing

(nm

), (l is by Kummer’s theorem, the number

of “carries” whenm andn−m are added in basep arithmetic), then

(4)(−1)l

pl

(n

m

)≡ n0!

m0!r0!· n1!

m1!r1!· · · ns!

ms!rs!(mod p),

wheren = n0 + n1p + · · · + nsps, m = m0 + m1p + · · · + msp

s, andr = n −m = r0 + r1p + · · · + rsp

s with 0 ≤ mi, ni, ri ≤ p − 1 for eachi = 0, 1, . . . , s.

Remark1. Numerous authors have asked whether there is an analogouscongruence modulopl to (4), for arbitraryl ≥ 1. In 1995 A. Granville[52, Theorem 1] gave a positive answer to this question (see the congruence(33)) in Subsection 3.2). �

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 6

The several proofs offered for Lucas’ theorem are primarilyof to types-algebraic and combinatorial. The well known algebraic proof of Lucas’theorem due to N.J. Fine [39] in 1947 is based on the binomial theorem forexpansion of(1 + x)n. This proof runs as follows. Since by Kummer’stheorem, the binomial coefficient

(pk

)is divisible by a primep for every

k = 1, 2, . . . , p− 1, by the binomial expansion it follows that

(1 +X)p ≡ 1 +Xp (mod p).

Continuing by induction, we have that for every nonnegativeintegeri

(1 +X)pi ≡ 1 +Xpi (mod p).

Write n andm in basep, so thatn =∑s

i=1 ni andm =∑s

i=1mi for somenonnegative integerss, n0, . . . , ns, m0, . . . , ms with 0 ≤ ni, mi ≤ p− 1 forall i = 0, 1, . . . , s. Then

n∑

m=0

(n

m

)Xm = (1 +X)n =

s∏

i=0

((1 +X)p

i)ni

≡s∏

i=0

(1 +Xpi

)ni

=

s∏

i=0

(ni∑

mi=0

(ni

mi

)Xmip

i

)(mod p)

=s∏

i=0

(p−1∑

mi=0

(ni

mi

)Xmipi

)

=

n∑

m=0

(s∏

i=0

(ni

mi

))Xm (mod p).

By comparing the coefficients ofXm on the left hand side and on theright hand side of the above congruence immediately yields Lucas’ theo-rem given by (1).

As an application of a counting technique due to M. Hausner in1983[55], in the same paper [55, Example 4] the author established another com-binatorial proof of (3). Another proof of the congruence (3)based on asimple combinatorial lemma is presented in 2005 by P.G. Anderson, A.T.Benjamin and J.A. Rouse in [6, p. 268] (see also [13]). Another two proofsof Lucas’ theorem, based on techniques from Elementary Number Theorywere obtained in 2010 by S.-C. Liu and J.C.-C. Yeh [83] and in 2012 by A.Laugier and M.P. Saikia [82].

The congruence (3) immediately yields

(5)

(np

mp

)≡(n

m

)(mod p)

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 7

since the same products of binomial coefficients are formed on the rightside of Lucas’s theorem in both cases, other than an extra

(00

)= 1.

A direct proof of the congruence (5), based on a polynomial method, isgiven in [133, Solution of Problem A-5, p. 173] as follows. Itis well knownthat

(pi

)≡ 0(mod p) for eachi = 1, 2, . . . , p− 1 (see (11)) or equivalently

that in the ringZp[x] we have(1 + x)p = 1 + xp, whereZp is the field ofthe integers modulop. Thus inZp[x],

np∑

k=0

(np

k

)xk = (1 + x)np = ((1 + x)p)n = (1 + xp)n =

n∑

j=0

(n

j

)xjp.

Since coefficients of like powers must be congruent modulop in the equalitynp∑

k=0

(np

k

)xk =

n∑

j=0

(n

j

)xjp

in Zp[x], we see that(np

mp

)≡(n

m

)(mod p) for m = 0, 1, . . . , n.

Further, notice that the Lucas’ congruence (3) easily follows by inductionon the sumr + s ≥ 0 using the base inductionr + s = 0 with r = s = 0satisfying via the congruence (5), and the Pascal formulas:

(np+ (r + 1)

mp+ s

)=

(np+ r

mp + (s− 1)

)+

(np+ r

mp+ s

)

and(

np + r

mp+ (s+ 1)

)=

(np+ (r − 1)

mp+ s

)+

(np + (r − 1)

mp+ (s+ 1)

).

Remark2. The Lucas’ congruence (3) also can be interpreted as a resultabout cellular automata (cf. Granville [52, Section 5]). Namely, Lucas’theorem can be interpreted as a two-dimensionalp-automaton (for a formaldefinition see [3]). �

2.2. Some consequences and extensions of Lucas’ theorem.Here, as al-ways in the sequel,p will denote any prime.

As noticed in 2011 by A. Nowicki [103, the congruences 7.3.1–7.33], ifn = n0 + n1p + · · ·+ nsp

s is thep-adic expansion of a positive integern,then for eachk = 0, 1, . . . , s

(6)

(n

pk

)≡ nk ≡

⌊n

pk

⌋(mod p),

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 8

holds, and consequently,

(7)

(n

p

)≡⌊n

p

⌋(mod p),

where⌊x⌋ is the greatest integer less than or equal tox.

Remark3. The congruence (7) is proposed by L.E. Clarke [26] in 1956as a problem which is solved in 1957 by P.A. Piza [108]. �

Moreover,if 0 ≤ r < pf and0 ≤ m < pf , then the Lucas’ congruence(3) immediately yields(see[103, the congruence 7.3.6])

(8)

(pf + r

m

)≡(r

m

)(mod p).

Furthermore,if 0 ≤ r < pf , 0 ≤ m < pf and a ≥ 0, then by Lucas’theorem(see[103, the congruence 7.3.7]),

(9)

(apf + r

m

)≡(r

m

)(mod p).

Moreover,if 0 ≤ r < pf andpf ≤ m, then by[103, the congruence 7.3.8],

(10)

(pf + r

m

)≡(

r

m− pf

)(mod p).

Lucas’ theorem immediately yields the following well knowncongru-ence:

(11)

(p

k

)≡ 0 (mod p),

wherep is a prime andk is an integer such that1 ≤ k ≤ p− 1.Furthermore,if p is a prime andf a positive integer, then by Lucas’

theorem for anyf ≥ 1 and 1 ≤ k ≤ pf − 1 we have(see, e.g.,[13,Theorem 24])

(12)

(pf

k

)≡ 0 (mod p).

Further,if p is a prime andn, m andk are positive integers withm ≤ n,then the congruence(5) by induction easily yields(see[96, Lemma 2.1])

(13)

(npk

mpk

)≡(n

m

)(mod p).

An alternative version of Lucas’ theorem was noticed in 1994by J. M.Holte [60, p. 60] (also see [61, p. 227]) as follows:If

B(m,n) :=

(m+ n

m

)=

(m+ n)!

m!n!,

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 9

then

(14) B(m,n) ≡ B(m div p, n div p)B(m mod p, n mod p) (mod p),

wherem div p is the integer quotient ofm byp andm mod p is the remain-der ofm by division byp. (similarly, for n instead ofm). It follows thatif n = n0 + n1p + · · · + nsp

s andm = m0 + m1p + · · · + msps, where

0 ≤ mi, ni ≤ p− 1 for eachi = 0, 1, . . . s, then

(15) B(m,n) ≡s∏

i=0

B(mi, ni) (mod p).

Consequently,p | B(m,n) if and only if p | B(mi, ni) for somei ∈{0, 1, . . . , s}.

Following Granville [52, Section 6], for an integer polynomial f(X) ofdegreed, define the numbers

(mn

)f

withm,n ∈ Z by the generating function

f(X)m =md∑

n=0

(m

n

)

f

Xn,

and let(mn

)f= 0 if n < 0 or n > md (note that

(mn

)f=(mn

)whenf(X) =

X + 1). Clearly, by Fermat little theorem,f(X)p ≡ f(Xp)(mod p), andusing this in 1995 A. Granville [52, Section 6, the congruence (24)] provedthe following generalization of the congruence (4):If p is a prime,m,nnonnegative integers such thatm = pl +m0, n = pt + n0, l, t,m0, n0 ∈ N

and0 ≤ m0, n0 ≤ p− 1, then

(16)

(m

n

)

f

≡d−1∑

k=0

( ⌊m/p⌋⌊n/p⌋ − k

)

f

(m0

n0 + kp

)

f

(mod p).

Notice that whenf(X) = X + 1 then the congruence (16) becomes(m

n

)≡(⌊m/p⌋⌊n/p⌋

)(m0

n0

)(mod p),

which is in fact the Lucas’s congruence (3).By using a congruence based on Burnside’s theorem, in 2005, T.J. Evans

[38, Theorem 3] proved the following extension of Lucas’ theorem involv-ing Euler’s totient functionϕ: If n ≥ 1,m,M,m0, r, R andr0 are nonneg-ative integers such thatm =Mn+m0, r = Rn+ r0, with 0 ≤ m0, r0 < n,then(17)∑

d|n

ϕ(nd

) d−1∑

j=−(d−1)

‖a‖d=

R−(j/d)

(M

a1

)· · ·(M

ad

)(m0

r0 + (n/d)j

)≡ 0 (mod n),

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 10

where the summation runs among all positive divisorsd of n.

Remark4. It was proved in [38, Corollary 3] that Lucas’ theorem easilyfollows from the congruence (17). �

3. LUCAS TYPE CONGRUENCES FOR PRIME POWERS

3.1. Wolstenholme type congruences.Notice that for any primep thecongruence (5) withn = 2 andm = 1 becomes

(2p

p

)≡ 2 (mod p),

whence by the identity(2pp

)= 2(2p−1p−1

)it follows that for any primep

(18)

(2p− 1

p− 1

)≡ 1 (mod p).

As noticed in 1, in 1819 Charles Babbage [9] (also see [52, Introduction] or[36, page 271]) showed that the congruence (18) holds modulop2, that is,for a primep ≥ 3 holds

(19)

(2p− 1

p− 1

)≡ 1 (mod p2).

Remark5. A combinatorial proof of the congruence (19) can be found in[126, Exercise 14(c) on page 118]. �

The congruence (19) was generalized in 1862 by Joseph Wolstenholme[142] as it is presented in the next section. Namely,Wolstenholme’s theoremasserts that

(20)

(2p− 1

p− 1

)≡ 1 (mod p3)

for all primesp ≥ 5.For a survey of Wolstenholme’s theorem see [93] and for its extensions

see [146] and [100].By Glaisher’s congruence [49, p. 323] (also see [93, Section6]), for any

positive integern and a primep ≥ 5 holds(np− 1

p− 1

)≡ 1 (mod p3),

which by the identity(npp

)= n

(np−1p−1

)yields [103, the congruence 7.1.5]

(21)

(np

p

)≡ n (mod p3).

In 1949 W. Ljunggren [19] generalized the congruence (21) asfollows(also see [10, Theorem 4], [52] and [126, Problem 1.6 (d)], and for a simple

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 11

proof see [123]):if p ≥ 5 is a prime,n andm are positive integers withm ≤ n, then

(22)

(np

mp

)≡(n

m

)(mod p3).

Remark6. Ljunggren’s congruence (22) is refined modulop5 in 2007 byJ. Zhao [145, Theorem 3.5]. �

Remark7. Note that the congruence (22) withm = 1 andn = 2 reducesto the Wolstenholme’s congruence (20). �.

Further, the congruence (22) is refined in 1952 by E. Jacobsthal [19] (alsosee [52]) as follows:if p ≥ 5 is a prime,n andm are positive integers withm ≤ n, then

(23)

(np

mp

)≡(n

m

)(mod pt),

wheret is the power ofp dividing p3nm(n −m) (this exponentt can onlybe increased ifp dividesBp−3, the(p− 3)rd Bernoulli number).

Remark8. In the literature, the congruence (23) is often calledJacobsthal-Kazandzidis congruence(see e.g., [27, Section 11.6, p. 380]). �

In 2008 C. Helou and G. Terjanian [56, the congruence (1) of Corollaryon page 490] refined the Jacobsthal’s result as follows (alsosee [27, Section11.6, Corollary 11.6.22, p. 381] for a stronger form)):If p ≥ 5 is a prime,n andm are positive integers withm ≤ n, then

(24)

(np

mp

)≡(n

m

)(mod pt),

wheret is the power ofp dividingp3m(n−m)(nm

).

By a problem N4 of Short list of 48th IMO 2006 [35],for every integerk ≥ 2, 23k divides the number

(25)

(2k+1

2k

)−(

2k

2k−1

)

but23k+1 does not.

3.2. Variations of Lucas’ theorem modulo prime powers. In 1991 D.F.Bailey [11, Theorem 4] proved thatif p is a prime,n andr are nonnegativeintegers ands a positive integer less thanp, then

(26)

(np

rp+ s

)≡ (r + 1)

(n

r + 1

)(p

s

)(mod p2).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 12

In the same paper [11, Theorem 5], the author extended the previous con-gruence as follows:if p ≥ 5 is a prime,0 ≤ m ≤ n, 0 ≤ r < p and1 ≤ s < p, then

(27)

(np2

mp2 + rp+ s

)≡ (m+ 1)

(n

m+ 1

)(p2

rp+ s

)(mod p3).

Remark9. Notice that Bailey’s proof of the congruence (27) (proof ofTheorem 5 in [10]) is deduced applying the Ljunggren’s congruence (22)(Theorem 4 in [10]) and a counting technique of M. Hausner from [55]. �

In 1992 D.F. Bailey [12, Theorem 2.1] generalized his congruence (27)modulo any prime power as follows:if p ≥ 5 is a prime,0 ≤ m ≤ n,s ≥ 1, anda0, a1, . . . , as−1 are nonnegative integers such that1 ≤ a0 < pand0 ≤ ak < p for everyk = 1, 2, . . . , s− 1, then

(nps

mps + as−1ps−1 + · · ·+ a1p+ a0

)

≡ (m+ 1)

(n

m+ 1

)(ps

as−1ps−1 + · · ·+ a1p+ a0

)(mod ps+1).

(28)

Remark10. If we puta = as−1ps−1+ · · ·+a1p+a0, then the congruence

(28) can be written as

(29)

(nps

mps + a

)≡ (m+ 1)

(n

m+ 1

)(ps

a

)(mod ps+1),

wherea is a positive integer less thanps which is not divisible byp. �.

Using a multiple application of Lucas’ theorem, in 2012 the author ofthis article [98, Theorem 1.1] proved the following similarcongruence to(29):

(30)

(nps

mps + a

)≡ (−1)a−1a−1(m+ 1)

(n

m+ 1

)ps (mod ps+1),

wherep is a prime,n,m, s anda are nonnegative integers such thatn ≥ m,s ≥ 1, 1 ≤ a ≤ ps − 1, anda is not divisible byp.

Remark11. The congruence (29) is an immediate consequence of thecongruence (30) (see [98, Corollary 1.2 and its proof]). �

In 1990 D.F. Bailey [10, Theorem 3] (cf. [97, Theorem withk = 2])proved the following result:If p is a prime,n,m, n0 andm0 are nonnegativeintegers, andn0 andm0 are both less thanp, then

(31)

(np2 + n0

mp2 +m0

)≡(n

m

)(n0

m0

)(mod p2).

Furthermore, in the same paper Bailey [10, Theorem 5] (cf. [97, Theoremwith k = 3]) extended the above result as follows:If p is a prime greater

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 13

than3 andn,m, n0 andm0 are nonnegative integers such thatn0 andm0

are less thanp, then

(32)

(np3 + n0

mp3 +m0

)≡(n

m

)(n0

m0

)(mod p3).

Kummer’s theorem given in Section 1, is useful in situationswhere thebinomial coefficient is divisible by a prime power. However,if the bino-mial coefficient is not congruent to zero modulo a prime, thenthe ques-tion remains for a way to simplify the expression. In 1995 A. Granville[52, Theorem 1] generalized Anton’s congruence (4) modulo prime pow-ers as follows. For a given integerk define(k!)p to be the product ofall integers less than or equal tok, which are not divisible byp. Sup-pose that prime powerpf and positive integersn andm are given withr := n − m ≥ 0. Write n = n0 + n1p + · · · + nsp

s in basep, and letNj be the least positive residue of⌊n/pj⌋(mod pf) for eachj ≥ 0 (so thatNj = nj + nj+1p + · · ·+ nj+f−1p

f−1); also make the corresponding defi-nitions formj ,Mj, rj, Rj . Let ej be the number of indicesi ≥ j for whichni < mi (that is, the number of “carries” when addingm andr in basep,on or beyond thejth digit). Then(33)1

pe0

(n

m

)≡ (±1)ef−1

(N0!)p(M0!)p(R0!)p

· (N1!)p(M1!)p(R1!)p

· · · (Ns!)p(Ms!)p(Rs!)p

(mod pf),

where(±1) is (−1) except ifp = 2 andf ≥ 3.Here, as usually in the sequel, we will consider the congruence rela-

tion modulo a prime powerpl extended to the ring of rational numberswith denominators not divisible byp. For such fractions we putm/n ≡r/s (mod pl) if and only if ms ≡ nr (mod pl), and the residue class ofm/n is the residue class ofmn′ wheren′ is the inverse ofn modulopl.

A result which gives readily an extension of Lucas’ theorem in the formof the congruence to prime power moduli is given in 1992 by A. Granville[51, Proposition 2] as follows:For each positive integerj, definenj to bethe least nonnegative residue of an integern modulopj . If p is a prime thatdoes not divide

(nm

), then

(34)

(n

m

)≡(⌊n/p⌋⌊m/p⌋

)(nf

mf

)/(⌊nf/p⌋⌊mf/p⌋

)(mod pf),

for any positive integerf .In particular,if

(nm

)is not divisible byp andm ≡ n(mod pf ), then by

(34) (also see[103, the congruence 7.1.16])

(35)

(n

m

)≡( ⌊n/p⌋⌊m/p⌋

)(mod pf).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 14

As observed in 1998 by D. Berend and J.E. Harmse [15, p. 34, congru-ence (2.2)],if a primep does not divide

(nm

)andn = n0+n1p+ · · ·+nsp

s,m = m0 +m1p + · · · +msp

s are thep-adic expansions ofn andm, theniterating the congruence(34), we find that

(36)

(n

m

)≡ P

Q(mod pf ),

where

P =

k−f+1∏

i=0

(ni + ni+1p+ . . .+ ni+f−1p

f−1

mi +mi+1p+ . . .+mi+f−1pf−1

)

and

Q =

k−f+1∏

i=1

(ni + ni+1p + . . .+ ni+f−2p

f−2

mi +mi+1p+ . . .+mi+f−2pf−2

).

The congruence (36) was established in 1991 independently by K. Davisand W. Webb [29, Theorem 3] (also see [85, p. 88, Theorem 5.1.2]), whichis there formulated as follows:If n = n0 + n1p + · · · + nsp

s, m = m0 +m1p+ · · ·+msp

s are thep-adic expansions ofn andm, andl < s, then(n

m

)≡(

n0 + n1p+ · · ·+ ns−1ps−1 + nsp

s

m0 +m1p+ · · ·+ms−1ps−1 +msps

)

≡(ns−l+···+nsps−l

ms−l+···+msps−l

)· · ·(n0+···+nlp

s−l

m0+···+mlps−l

)(ns−l+1+···+ns−1ps−l−1

ms−l+1+···+ms−1ps−l−1

)· · ·(n0+···+nl−1ps−l−1

m0+···+ml−1ps−l−1

) (mod pl).

(37)

If a = a0+a1p+· · ·+ak−1pk−1+akp

k andb = b0+b1p+· · ·+bk−1pk−1+bkp

k

are thep-adic expansions ofa andb such thatbk > ak, then we define(a0 + a1p+ · · ·+ ak−1p

k−1 + akpk

b0 + b1p+ · · ·+ bk−1pk−1 + bkpk

)= p

(a0 + a1p+ · · ·+ ak−1p

k−1

b0 + b1p+ · · ·+ bk−1pk−1

).

Remark12. For help in understanding the above result concerning thecongruence (37), we offer the following example [85, p. 88]:

(386

154

)=

(3 · 112 + 2 · 11 + 1

112 + 3 · 11

)≡(3·11+211+3

)(2·11+13·11

)(23

) (mod 112)

≡(3 · 11 + 2

11 + 3

)(1

0

)≡(35

14

)(mod 112). �

In 2005 A.D. Loveless [85, p. 88] noticed that the above result concern-ing the congruence (37) can be used to simplify general classes of congru-ences modulo prime powers involving binomial coefficients.In particular,

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 15

Loveless [85, p. 88, Theorem 5.1.3]) proved thatif p is a prime,s andn arepositive integers withn ≤ ps, then

(38)

(ps

n

)≡{

0 (mod ps) if n 6≡ 0 (mod p)(ps−1

n/p

)(mod ps) if p | n.

A similar result was earlier directly proved in 1980 by P.W. Haggard andJ.O. Kiltinen [54, p. 398, Theorem]. This result asserts that if p is a prime,l andf are positive integers withf ≥ l − 1 and0 ≤ n ≤ pf , then

(39)

(pf

n

)≡{

0 (mod pl) if n 6≡ 0 (mod pf−l+1)(pl−1

i

)(mod pl) if n = i · pf−l+1.

Using the congruence (37), in 1993 K. Davis and W. Webb [30] generalizedBailey’s results concerning the congruences (31) and (32) for any moduluspk with p ≥ 5 andk ≥ 1. They proved [30, Theorem 3] thatif p is anyprime,k, n,m, a, b ands are positive integers such that0 < a, b < ps, then

(40)

(npk+s + a

mpk+s + b

)≡(npk

mpk

)(a

b

)(mod pk+1).

Remark13. Notice that under the same assumption preceding the congru-ence (40), and if

(npk+s+ampk+s

)6≡ 0(mod p), then the congruence (40) can be

obtained by iteratings times the Granville’s congruence (34). Notice alsothat the condition

(npk+s+ampk+s

)6≡ 0(mod p) is by Lucas’ theorem equivalent

to the following two conditions:(nm

)6≡ 0(mod p) and

(ab

)6≡ 0(mod p). �

Further, by repeated application of the congruence (40), and using Ljung-gren’s congruence (22), we find thatunder the same assumptions precedingthe congruence(40) [30, Corollary 1]for any primep > 3,

(41)

(npk+s + a

mpk+s + b

)≡(np⌊k/3⌋

mp⌊k/3⌋

)(a

b

)(mod pk+1).

In particular, the congruence (41) withs = 1 andk − 1 ≥ 0 instead ofkimplies thatfor each primep ≥ 5 and for all integersk ≥ 1, n ≥ 0, a andb with 0 ≤ a, b < p

(42)

(npk + a

mpk + b

)≡(np⌊(k−1)/3⌋

mp⌊(k−1)/3⌋

)(a

b

)(mod pk).

Furthermore,the congruence(42) with ⌊k/2⌋ instead of⌊(k − 1)/3⌋ issatisfied forp = 2, and the congruence(42) with ⌊(k − 1)/2⌋ instead of⌊(k − 1)/3⌋ is also satisfied forp = 3.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 16

Remark14. As noticed above, a proof of the congruence (41) given byDavis and Webb is based on their earlier result from [29] given by the con-gruence (41). However, this result together with related proof is slightlymore complicated. In 2012 the author of this article [97, Theorem] gavea simple induction proof of the congruence (42) which uses only the usualproperties of binomial coefficients. �

Adapting Fine’s method [39], in 1988 R.A. Macleod [88, Theorem 2]proved the following variation of Lucas’ theorem:Letp be a prime, letr bea positive integer, and let

M =

k∑

i=0

Mipir, with 0 ≤Mi < pr for all i = 0, 1, . . . , k.

Then for every nonnegative integerN such that0 ≤ N ≤M

(43)

(M

N

)≡∑(

pr−1M0

N0

)(pr−1M1

N1

)· · ·(pr−1Mk

Nk

)(mod pr),

where the summation ranges over allk + 1-tuples(N0, N1, . . . , Nk) suchthat

pr−1N =

k∑

i=0

Nipir, with 0 ≤ Ni < pr−1Mi for all i = 0, 1, . . . , k.

Quite recently, in 2014 E. Rowland and R. Yassawi [115, Section 5, The-orem 5.3] established a new generalization of Lucas’ theorem to primepowers as follows:Let p be a prime, letf be a positive integer and letD = {0, 1, . . . , pf − pf−1}. If n = n0 + n1p + · · · + nsp

s andm =m0 +m1p+ · · ·+msp

s are thep-adic expansions of nonnegative integersn andm, then(

n

m

)≡

(i0,...,il)∈Dl+1

(j0,...,jl)∈Dl+1

(−1)n−i+∑l

h=0 ih

(pf−1 − 1

n− i

)(n− i

m− j

)

×l∏

h=0

(pf − pf−1

ih

)(ihjh

)(mod pf),

(44)

wherei =∑l

h=0 ihph andj =

∑lh=0 jhp

h.

Remark15. Note thati =∑l

h=0 ihph andj =

∑lh=0 jhp

h are represen-tations of integersi andj in basep with an enlarged digit setD rather thanthe standard digit set{0, 1, . . . , p− 1}. �

Remark16. E. Rowland and R. Yassawi [115, Section 5] showed thata broad range of multidimensional sequences possess “Lucasproducts”modulo a primep. Furthermore, in 2009 K. Samol and D. van Straten

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 17

[117, Proposition 4.1] established the Lucas type congruence for a sequencewhose terms are constant terms ofP (x)n for certain Laurent polynomialsP (x). �

3.3. Characterizations of Wolstenholme primes.A primep is said to beaWolstenholme primeif it satisfies the congruence

(2p− 1

p− 1

)≡ 1 (mod p4),

or equivalently,

(45)

(2p

p

)≡ 2 (mod p4).

The two known such primes are 16843 and 2124679, and R.J. McIntosh andE.L. Roettger reported in [91] that these primes are only twoWolstenholmeprimes less than109. However, McIntosh in [90] conjectured that there areinfinitely many Wolstenholme primes (for more information see [94]). Bythe well known result of J.W.L. Glaisher in 1900 [49, p. 323] (also see [95,the congruence (1.2)]),

(46)

(2p− 1

p− 1

)≡ 1− 2

3p3Bp−3 (mod p4),

whereBk (k = 0, 1, 2, . . .) are Bernoulli numbers defined by the generatingfunction [71]

∞∑

k=0

Bkxk

k!=

x

ex − 1.

The congruence (46) shows that a primep is a Wolstenholme prime if andonly if p divides the numerator ofBp−3, the(p− 3)rd Bernoulli number.

As an application of the congruences (42) withk = 4 and Jacobsthal’scongruence (23), we can obtain the following characterization of Wolsten-holme primes given in 2012 by the author of this article [97, Proposition]:The following statements about a primep ≥ 5 are equivalent.

(i) p is a Wolstenholme prime;(ii) for all nonnegative integersn andm the congruence

(47)

(np

mp

)≡(n

m

)(mod p4)

holds;(iii) for all nonnegative integersn,m, n0 andm0 such thatn0 andm0

are less thanp,

(48)

(np4 + n0

mp4 +m0

)≡(n

m

)(n0

m0

)(mod p4).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 18

4. THE LUCAS PROPERTY AND THEp-LUCAS PROPERTY

4.1. The Lucas property and the double Lucas property. In 1992 R.J.McIntosh [89] proposed the following definition:

Definition. The integer sequence(an)n≥0 has theLucas propertyif a0 =1, and for every primep, everyn ≥ 0, and everyj ∈ {0, 1, . . . , p − 1} thecongruence

(49) apn+j ≡ anaj (mod p)

holds. �

Remark17. (cf. [1, p. 152, Remark 6.1]). Takingn = j = 0 in the con-gruence (49) givesa0 ≡ a20(mod p). This yields that eithera0 ≡ 0(mod p)or a0 ≡ 1(mod p). In the first case, takingn = 0 andj ∈ {0, 1, . . . , p− 1}givesaj ≡ 0(mod p); henceapn+j ≡ anaj ≡ 0(mod p) for all n’s andj’s.This means thatan is a zero sequence modulop. What precedes impliesthat such a sequence either satisfiesan = 0 for all n ≥ 0 or a0 = 1. �

An analogous definition of double Lucas property is given also by McIn-tosh [89] as follows:

Definition. The functionL : N × N → Z has thedouble Lucas propertyif L(n,m) = 0 for all n < m, and for every primep, everyn,m ≥ 0, andeveryr, s with 0 ≤ r, s ≤ p− 1 the congruence

(50) L(np + r,mp + s) ≡ L(n,m)L(r, s) (mod p)

holds. �

Notice that Lucas’ theorem (the congruence (3)) and the congruence (14)show that both functionsC(n,m), B(n,m) : N × N → Z defined asC(n,m) =

(nm

)andB(n,m) =

(n+mm

)have the double Lucas property.

McIntosh [89] presents various properties of the functionL(n, k) and theirconnection with tre Lucas property. A typical result is as follows: If L(n,m)has the double Lucas property, then the functionF (n) =

∑nm=0 L(n,m)

has the Lucas property.In 1999 J.-P. Allouche [1, Proposition 7.1] proved the following result:

Let m be a positive integer, lete1 = 2, e2, . . . , em be integers such thatej ≤ ej+1 ≤ 2ej for j = 1, 2, . . . , m− 1, and letr1, r2, . . . , rm be positiveintegers. Then the sequence(un)n≥0 defined by

(51) un =

(2n

n

)r1(e2n2n

)r2(e3ne2n

)r3

· · ·(emn

em−1n

)rm

has the Lucas property.In particular,if ej+1−ej = 1 for all j = 1, 2, . . . , m−1, andr1, r2, . . . , rm

are positive integers, then the above result implies that the sequence(un)n≥0

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 19

defined as

un =

(2n

n

)r1(3nn

)r2

· · ·((m+ 1)n

n

)rm

has the Lucas property(see[89]).TheApery numbersA1(n) andA2(n) defined as

A1(n) =

n∑

k=0

(n

k

)2(n+ k

k

)2

, A2(n) =

n∑

k=0

(n

k

)2(n+ k

k

), n = 0, 1, . . . ,

arose in Apery’s proof in 1979 of the irrationality ofζ(3) [8]. (A1(n))n≥0

and (A1(n))n≥0 are Sloane’s sequences A005259 and A005258 in [124],respectively.

The Apery numbers modulo a prime were studied in 1982 by I. Gesselwho proved [47, Theorem 1] the following result:If n = n0 + n1p+ · · ·+nsp

s is thep-adic expansion ofn, then

(52) A1(n) ≡s∏

i=0

A1(ni) (mod p).

In other words, the sequence(A1(n))n≥1 has the Lucas property.Similarly, the sequence(A2(n))n≥0 satisfies the Lucas property(see[31]).In 2008 Y. Jin, Z-J. Lu and A.L. Schmidt [72, (ii) of Lemma 2] proved

that the sums of powers of binomial coefficients have the Lucas property,that is:For a positive integers, let (a(s)n )n≥0 be a sequence defined as

a(s)n =

n∑

k=0

(n

k

)s

, n = 0, 1, 2, . . . .

Then for every primep, everyn ≥ 0, and everyj ∈ {0, 1, . . . , p − 1} thecongruence

(53) a(s)pn+j ≡ a(s)n a

(s)j (mod p)

holds.

Remark18. The above result implies that the residues of Pascal’s trianglemodulop have a self-similar structure (see, e.g., [42], [52, Section 5] and[141]). �

For a primep and a positive integerk, in 1994 M. Razpet [111] con-sidered thepk × pk matrixA(k, p) = [ai,j(k, p)]

0≤j≤pk−10≤i≤pk−1

, whose the entry

ai,j(k, p) is defined as the remainder of the division of(ij

)by p. In partic-

ular, for k = 1 we writeA(p) = A(1, p) = [ai,j(p)]0≤j≤p−10≤i≤p−1 . M. Razpet

[111] noticed that for everyk ≥ 1 and every primep, the matrixA(k, p) isthek-fold tensor (or Kronecker) product of the matrixA(p) by itself in the

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 20

field Zp, that is,A(k, p) = A(p)⊗ A(p) · · · ⊗ A(p)︸ ︷︷ ︸k

= A(p)⊗k. Note that

matrix indices start at index pair(0, 0). This is an algebraic and “square”representation of the oft-noted self-similarity structure of Pascal’s triangle(see, e.g., [58] and [141]).

Furthermore, as noticed in [111, p. 378], by Lucas’ theorem we have

(54) ai,j(k, p) ≡ ai0,j0(p)ai1,j1(p) · · ·aik−1,jk−1(p) (mod p),

where0 ≤ i, j ≤ pk − 1, i = i0 + i1p+ · · ·+ ik−1pk−1 andj = j0 + j1p+

· · ·+ jk−1pk−1 with 0 ≤ il, jl ≤ p− 1 for all l = 0, 1, . . . , k − 1.

Remark19. In [109] M. Prunescu pointed out that Pascal’s triangle mod-ulopk is not a limit of tensor powers of matrices ifk ≥ 2. However, Pascal’striangle modulopk arep-automatic, and consequently can be produced bymatrix substitution and are projections of double sequences produced bytwo-dimensional morphisms (see [4]). �

In 2003 D. Berend and N. Kriger [14, Theorem 5] proved thatthere existuncountably many infinite matricesA = [ai,j ]

∞m,n=0 satisfying the double

Lucas property, that is the congruences

(55) am,n ≡k∏

i=0

ami,ni(mod p)

are satisfied for every primep and all nonnegative integersm andn withp-adic expansionsm =

∑ki=0mip

i andn =∑k

i=0 nipi.

In 1998 N.J. Calkin [21] investigated divisibility properties for sums ofpowers of binomial coefficientsfn,a defined as

fn,a =n∑

k=0

(n

k

)a

,

wheren anda are nonnegative integers. Thenfn,0 = n + 1, fn,1 = 2n

andfn,2 =(2nn

). The sequences(fn,a)n≥0 for a = 3, 4, 5, 6 are Sloane’s

sequences A000172 (Franel numbers), A005260, A005261, A069865 in[124], respectively. Calkin [21, Lemma 4] proved thatfor every positiveintegera, the sequence(fn,a)n≥0 has the Lucas property. This means that ifp is a prime and ifn = n0 + n1p+ · · ·+ nsp

s is thep-adic expansion ofn,then

(56) fn,a ≡s∏

i=0

fni,a (mod p).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 21

Calkin [21, p. 21] also noticed thatfor anya ∈ {1, 2, . . .} the sequence(hn,a)n≥0 defined as

hn,a =n∑

k=0

(−1)k(2n

k

)a

,

also has the Lucas property.For a positive integern the central trinomial coefficientTn is the largest

coefficient in the expansion(1 + x + x2)n (Sloane’s sequence A002426 in[124]). It is easy to expressTn in terms of trinomial coefficients as

Tn =∑

k≥0

(n

k, k, n− 2k

),

where we use the convention that if any multinomial coefficient has a neg-ative number on the bottom then the coefficient is zero. In 2006 E. Deutschand B.E. Sagan [33] proved that the sequence(Tn)n≥0 has the Lucas prop-erty. Namely, by [33, Theorem 4.7] ifp is a prime andn = n0 + n1p +· · ·+ nsp

s is a positive integer with0 ≤ ni ≤ p− 1 for all i = 0, 1, . . . , s,then

(57) Tn ≡s∏

i=0

Tni(mod p).

Furthermore, E. Deutsch and B.E. Sagan [33, Theorem 4.4] proved thefollowing result for central binomial coefficients

(2nn

)(Sloane’s sequence

A000984 in [124]):Let p be a prime and letn = n0 + n1p + · · · + nsps

be a positive integer with0 ≤ ni ≤ p − 1 for all i = 0, 1, . . . , s. Foreveryj ∈ {0, 1, . . . , p− 1} let δp,j(n) be the number of elements of the set{n0, n1, . . . , ns} equal toj. Then(58)(2n

n

)≡{ ∏

j

(2jj

)δp,j(n)(mod p) if ni ≤ p/2 for all i = 0, 1, . . . s,

0 (mod p) otherwise,,

where the summation ranges over allj ∈ {0, 1, . . . , p−1} such thatδp,j(n) >0.

In 2009 M. Chamberland and K. Dilcher [25] studied the divisibilityproperties of the sumsu(n) defined as

u(n) =n∑

k=0

(−1)k(n

k

)(2n

k

), n = 0, 1, 2, . . . .

Under this notation, the authors proved [25, Theorem 2.2]that for everyprimep ≥ 3 and all integersm ≥ 0 andr such that0 ≤ r ≤ (p− 1)/2 we

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 22

have

(59) u(mp+ r) ≡ u(m)u(r) (mod p).

As an application, the authors proved [25, Corollary 2.1] that for everyprime p ≥ 3 and every integern = n0 + n1p + · · · + nsp

s with 0 ≤ni ≤ (p− 1)/2 for eachi = 0, 1, . . . , s, we have

(60) u(n) ≡ u(n0)u(n1) · · ·u(ns) (mod p).

Similarly, if the sumsw(n) are defined as

w(n) =n−1∑

k=0

(−1)k(2n− 1

k

)(n− 1

k

), n = 0, 1, 2, . . . ,

then by [25, Corollary 2.2],for all primesp ≥ 3 and positive integersmandr with (p+ 1)/2 ≤ r ≤ p− 1

(61) u(mp+ r) ≡ w(m+ 1)u(r) (mod p).

Remark20. We point out that the Lucas property holds for a generalfamily of sequences considered in 2006 by T.D. Noe [102]. �

For all nonnegative integersi andj let w(i, j|a, b, c) denote the numberof all paths in the plane from(0, 0) to (i, j) with steps(1, 0), (0, 1), (1, 1),and with positive integer weightsa, b, c, respectively. The explicit formulafor w(i, j|a, b, c) was obtained by several authors by using combinatorialarguments (see, e.g., [43]):

w(i, j|a, b, c) =∑

k≥0

(k

i

)(i

k − j

)ak−jbk−ici+j−k.

Actually, k in the above sum runs frommax{i, j} to i + j. In the casea = b = c = 1, we have even theDelannoy numberswhich count the usual,unweighted lattice paths from the point(0, 0) to the point(i, j) with stepsalong the vectors(1, 0), (0, 1) and(1, 1). If i = j = n, then the numbersw(n, n|1, 1, 1), n = 0, 1, 2, . . . are called thecentral Delannoy numbers(Sloane’s sequence A001850 in [124]).

In 2002 M. Razpet [112, Theorem 1] proved the following double Lucasproperty ofw(i, j|a, b, c): Letp be a prime and letα, β, γ, δ be nonnegativeintegers where0 ≤ β < p and0 ≤ δ < p. Then the congruence

(62) w(αp+ β, γp+ δ|a, b, c) ≡ w(α, γ|a, b, c)w(β, δ|a, b, c) (mod p)

holds for all positive integersa, b, c.

Remark21. Razpet [112] notice that the congruence (62) is particularlytrue for theDelannoy numbersD(i, j) := w(i, j|1, 1, 1) as proven in an-other way in 1990 by M. Razpet [110] and by M. Sved and R.J. Clarke[132] (see also [33] and [37]). �

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 23

In 2004 H. Pan [106, Theorem 1] proved the following result:Supposeλ(x1, . . . , xn) =

∑Φ 6=I⊆{1,...,n} αI

∏i∈I Xi, αI ∈ F, is a polynomial over

the finite fieldF with q elements. Letwλ(k1, . . . , kn) be the coefficient of∏ni=1X

kii in the formal power series 1

1−λ(x1,...,xn). Thenwλ satisfies the

double Lucas property, i.e., for any nonnegative integersa1, . . . , an and0 ≤ b1, . . . , bn < q,

(63) wλ(a1q + b1, . . . , anq + bn) = wλ(a1, . . . , an)wλ(b1, . . . , bn).

Remark22. If p is a prime andF is the fieldZp = {0, 1, . . . , p− 1}, thenthe equality “=” in (63) becomes≡ 0(mod p). �

4.2. Further Lucas type congruences.For nonnegative integersn andkStirling numbers of the second kind

{nk

}(Sloane’s sequence A008277 in

[124]) are recursively defined as:

{n

k

}=

1 if n = 0, k = 0,0 if n > 0, k = 0,0 if n = 0, k > 0,k{n−1k

}+{n−1k−1

}if n > 0, k > 0.

{nk

}presents the number of ways of partitioning a set ofn elements intok

nonempty sets (i.e.,k set blocks). They (as well as Stirling numbers of thefirst kind defined below) are named after James Stirling, who introducedthem in 1730 [127].

In 1988 M. Sved [131, p. 61, Theorem] showed the following result: Letn andm be nonnegative integers, and letp be a an odd prime such thatpdoes not dividem. Put

n′ =

⌊pn− p⌊m/p⌋ − 1

p− 1

⌋,

and letn′ =∑h

i=0 n′ip

i andm =∑h

i=0mipi be the expansions ofn′ andm

to basep. Then

(64)

{n

m

}≡{n′0

m0

} h∏

j=1

(n′j

mj

).

In 2000 R. Sanchez-Peregrino [118, Proposition 3.1] proved thatif n,m, rands are nonnegative integers such that0 ≤ s ≤ r ≤ p− 1 andm ≤ n ≤p− 1, then(65){np+ r

mp + s

}≡{n−m+ r

s

}(n

m

)+

{n−m+ r + 1

s + p

}(n

m− 1

)(mod p).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 24

Notice also that under the hypothesis thatr + n − m + 1 < s + p, thecongruence(65) reduces to

(66)

{np+ r

mp+ s

}≡{n−m+ r

s

}(n

m

)(mod p).

Furthermore, by [118, Proposition 4.1],if r, s, a andf are nonnegativeinegers, then(67){

apf + r

s

}≡

i0+i1+···+if

(a

i0, i1, . . . , if

){r + i0

s−∑fl=1 ilp

f

}(mod p).

Remark23. As noticed in [118, Remark 3.1], in the caser < p thecongruence (65) gives the formulas (4.17) and (4.18) of F.T.Howard [67]from 1990. �

For nonnegative integersn andk Stirling numbers of the first kind[nk

]

(Sloane’s sequence A008275 in [124]) are defined by the recurrence relation

[n

k

]=

1 if n = 0, k = 0,0 if n > 0, k = 0,0 if n = 0, k > 0,(n− 1)

[n−1k

]+[n−1k−1

]if n > 0, k > 0.

The absolute value of[nk

](Sloane’s sequence A094638 in [124]) denotes,

as usual, the number of permutations ofn elements which contain exactlyk permutation cycles.

In 1993 R. Peele, A.J. Radcliffe and H.S. Wilf [107, Proposition 2.1]proved the following analogue of Lucas’ theorem for the numbers

[nk

]: Let

p be a prime and letn andk be integers with1 ≤ k ≤ n. Letn′ = ⌊n/p⌋andn0 = n− n′p. Further, define integersi andj as follows:

k−n′ = j(p−1)+i with 0 ≤ i < p−1 if n0 = 0; 0 < i ≤ p−1 if n0 > 0.

Then

(68)

[n

k

]≡ (−1)n

′−j

(n′

j

)[n0

i

](mod p).

For a nonnegative integerk let Jk(z) be the —it Bessel function of thefirst kind. Put

fk(z) =Jk(2

√z)

zk/2=

∞∑

i=0

(−1)izi

i!(i+ k)!.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 25

Furthermore, define the polynomialui(k; x) by means of

k!fk(xz)

fk(z)=

∞∑

i=0

ui(k; x)zi

i!(i+ k)!.

Certain Lucas type congruences forwi(x) = ui(0; x) and the integerswi =wi(0)with i = 0, 1, 2 . . . ,were derived by L. Carlitz [22] in 1955, and an in-teresting application was presented ((wn)n≥0 is Sloane’s sequence A000275).In 1987 F.T. Howard [66, Theorem 1] proved a more general result as fol-lows: Letk, n ands be nonnegative integers, and letp be a prime such thatp ≥ 2k ands < p − 2k. Then the numbersui(k) := ui(k; 0) are integral(mod p) for all i = 0, 1, 2 . . . ; in particular, un(0) andun(1) are positiveintegers for alln = 0, 1, 2 . . .. Furthermore, for any fixedk ≥ 0 and everyprimep the congruence

(69) unp+s(k) ≡ us(k) · wn (mod p)

holds for alln ≥ 0 and0 ≤ s ≤ n− 1.With the assumptions of the above statement, ifm is a nonnegative integer

with the expansionm =∑s

i=0mipi to basep satisfyingm0 < p− 2k, then

the congruence(70) with k = 0 implies Carlitz’s result[22] from 1955which asserts that the sequence(wn)n≥0 has the Lucas property, i.e.,

(70) wm ≡s∏

i=0

wmi(mod p),

Furthermore,the following two congruences are satisfied[66, p. 306,Corollary and Theorem 2]:

(71) um(k) ≡ um0(k)

s∏

i=1

wmi(0) (mod p),

and

(72) unp−k(k) ≡ (−1)ku0(k) · wn(0) (mod p).

Let p be a prime and letn, r, l anda be positive integers. Following Z.-W.Sun and D. Wan [130], thenormalized cyclotomicψ-coefficientis definedas

(73)

{n

r

}

l,pa:= p

{

⌊n−pa−1−lpa

ϕ(pa)

{

⌋ ∑

k≡r ( mod pa)

(−1)k(n

k

)((k − r)/pa

l

).

In 2008 Z.-W. Sun and D. Wan [130, Theorem 1.1] proved thatif p is anyprime,r is an integer anda, l, n, s, t are positive integers witha ≥ 2 and

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 26

s, t < p, then

(74)

{pn+ s

pr + t

}

l,pa+1

≡ (−1)t(s

t

){n

r

}

l,pa(mod p).

It is noticed in [130, Remark 1.1] that in the casel = 0 the congruence(74) is equivalent to Theorem 1.7 in [129] due to Z.-W. Sun andD.M. Davisin 2007. Under the same conditions preceeding the congruence (74), Sunand Davis [129, Theorem 1.7] proved the following congruence of Lucas’type:

1

⌊n/pa−1⌋!∑

k≡r ( mod pa)

(−1)pk(pn+ s

pk + t

)(k − r

pa−1

)l

≡ 1

⌊n/pa−1⌋!∑

k≡r ( mod pa)

(−1)k(n

k

)(s

t

)(k − r

pa−1

)l

(mod p).

(75)

J. Boulanger and J.-L. Chabert [18] have extended Lucas’ theorem toLinear Algebra and Even Topology. Their result can be brieflyexposed asfollows. LetV be a discrete valuation domain with finite residue field. De-note byK the quotient field ofV , byv the corresponding valuation ofK, bym the maximal ideal ofV , and byq the cardinality of the residue fieldV/m.We denote byK, V andm the completions ofK, V andm, respectively,with respect to them-adic topology and we still denote byv the extensionof v to K. Consider the ringInt(V ) of integer-valued polynomials onV ,that is,

Int(V ) = {f ∈ K[X ] : f(V ) ⊆ V }.A basisCn(X) of theV -moduleInt(V ) can be constructed as follows [20,Chapter II,§2 ]. We choose a generatort of m and a setU = {u0 =0, u1, . . . , uq−1} of representatives ofV modulom. It is known that eachelementx of V has a uniquet-adic expansion

x =

∞∑

j=0

xjtj with xj ∈ U for each j ∈ N.

We now construct a sequence(un)n≥0 of elements ofV which will re-place the sequence of nonnegative integers. Takingq as the basis of the nu-meration, that is, writing every positive integern in the formn =

∑ki=0 niq

i

with 0 ≤ ni < q for eachi = 0, 1, . . . , k, we extend the sequence(uj)0≤j<k

in the following way:

un = un0 + un1t + un2t2 + · · ·+ unk

tk.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 27

We then replace the binomial polynomials(X

n

)=X(X − 1)(X − 2) · · · (X − n + 1))

n!

(which form a basis of theZ-moduleInt(Z) = {f ∈ Q[X ] : f(Z) ⊆ Z} ofinteger-valued polynomials onZ) by the polynomials defined as

Cn(X) =

n−1∏

k=0

X − ukun − uk

, n = 1, 2, . . . , and C0 = 1.

Then by [20, Theorem II.2.7], the sequence of polynomials(Cn(X))n≥0

form a basis of theV -module Int(V ). In 2001 J. Boulanger and J.-L.Chabert [18, Theorem 2.2] proved the following “generalized Lucas’ theo-rem”: If

n = n0 + n1q + . . .+ nkqk

is theq-adic expansion of a positive integern, and if

x = x0 + x1t+ . . .+ xjtj + . . .

is thet-adic expansion of an elementx of V , then

(76) Cn(x) ≡ Cn0(x0)Cn1(x1) · · ·Cnk(xk) (mod m).

Remark24. Notice also that in 1993 N. Zaheer [144] generalized Lucas’theorem to vector-valued abstract polynomials in vector spaces. �

5. LUCAS TYPE THEOREMS FOR SOME GENERALIZED BINOMIAL

COEFFICIENTS

5.1. Generalized binomial coefficients and related Lucas type congru-ences.LetA andB be nonzero integers. TheLucas sequenceu0, u1, u2, . . .is defined recursively as

(77) u0 = 0, u1 = 1 and un+1 = Aun − Bun−1 for n = 1, 2, 3, . . . .

The companion sequence of Lucas sequence(un)n≥0 is the sequence(vn)n≥0

recursively defined as

(78) v0 = 2, v1 = A and vn+1 = Avn −Bvn−1 for n = 1, 2, 3, . . . .

It is well known that for alln = 0, 1, 2, . . .

un =αn − βn

α− βand vn = αn + βn,

where

α =A+

√∆

2, β =

A−√∆

2and ∆ = A2 − 4B.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 28

In fact,α andβ are roots of the characteristic equationx2 − Ax + B = 0.Note that forA = 1, B = −1 the terms of the sequence(un)n≥0 definedby (77) are the well-knownFibonacci numbersFn defined recursively asF0 = 0, F1 = 1 and

Fn+1 = Fn + Fn−1 for n ≥ 1.

Fibonacci numbers are in fact the Lucas sequence(un)n≥0 given by (77)with u0 = 0 andu1 = 1.

Similarly, theLucas numbersLn are defined byL0 = 2, L1 = 1 and

Ln+1 = Ln + Ln−1 for n ≥ 1.

Fibonacci numbersFn and Lucas numbersLn are given as Sloane’s se-quences A000045 and A000032 in [124], respectively.

Let a := (an)n≥0 be a sequence of real or complex numbers such thatan 6= 0 for all n ≥ 1. Thea-nomial coefficients(or thegeneralized binomialcoefficients) (associated to the sequencea) are defined by[n

k

]

a

=anan−1 · · · a1

(akak−1 · · · a1)(an−kan−k−1 · · ·a1)for n ≥ 2 and 1 ≤ k ≤ n−1,

and [n

0

]

a

=

[n

n

]

a

= 1 for n ≥ 0.

This definition was suggested in 1915 by Georges Fontene in his one-pagenote [41]. A number of authors have considered different classes of gener-alized binomial coefficients

[nk

]a

(usually, whena := (an)n≥0 is an integersequence). Related investigations were done in 1913 by R.D.Carmichael[24], in 1936 by M. Ward, [136], in 1967 by R.D. Fray [42] and V.E. Hog-gatt [59], in 1969 by H.W. Gould [50], and later by several authors ([61],[62], [77], [79], [102], [134] and [135]). For example, in 1989 D.E. Knuthand H.S. Wilf [79, Proposition 3] generalized Kummer’s theorem for thea-nomial coefficients

[m+km

]a, wherea = (an)n≥1 is a sequence of positive

integers. Consequently, they obtained [79, Theorems 1 and 2] Kummer’stheorem for the Gaussianq-nomial coefficients

[m+km

]q

whereq > 1 is an

integer and for the Fibonomial coefficients[m+km

]F

defined below, respec-tively.

In general, even if the all terms of a sequencea = (an)n≥0 are integers,[nk

]a

may not be integers. In 1913 R.D. Carmichael [24, page 40] provedthat if the sequencea := (an)n≥1 of positive integers is defined recursivelyas

a1 = a2 = 1, and an+1 = can + dan−1 for n = 2, 3, 4, . . . ,

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 29

wherec andd are integers, then the alla-nomial coefficients are integers.For a more general result see Remark 28.

If u := (un)n≥0 is the Lucas sequence defined by (77), and ifA 6= ±1 orB 6= 1, thenu1, u2, . . . are nonzero (see, e.g., [69]), and so arev1 = u2/u1,v2 = u4/u2, . . ., wherev := (vn)n≥0 is the companion sequence of thesequence(un)n≥0 given by (78). In the case whenA2 = B = 1, then asnoticed in [69]un = 0 if and only if 3 | n. If vn = 0, thenu2n = unvn = 0;hence3 | n andun = 0, which is impossible sincev2n − ∆u2n = 4Bn (cf.[68]). Thusv0, v1, v2, . . . are all nonzero.

If f A 6= ±1 orB 6= 1 theLucasu-nomial coefficient[nk

]u

with 1 ≤ k ≤ nis the generalized binomial coefficient associated to the Lucas sequenceu := (un)n≥0 defined by (77), that is,[n

k

]

u

=unun−1 · · ·u1

(ukuk−1 · · ·u1)(un−kun−k−1 · · ·u1)for n ≥ 2 and 1 ≤ k ≤ n−1,

and[n0

]u=[nn

]u= 1 for all n ≥ 0.

In the sam way we define thev-nomial generalized binomial coefficient[nk

]v, wherev := (vn)n≥0 is the companion sequence of the Lucas sequence

(un)n≥0 defined by (78).

Remark25. In the caseA = 2 andB = 1, (77) yieldsun = n for alln = 0, 1, 2, . . ., and hence

[nk

]u

is exactly the binomial coefficient(nk

). �

Similarly, theFibonomial coefficients(or Fibonacci coefficients) are de-fined as the generalized binomial coefficients associated tothe sequence(Fn)n≥ of Fibonacci numbers, that is,[n

k

]

F

=FnFn−1 · · ·F1

(FkFk−1 · · ·F1)(Fn−kFn−k−1 · · ·F1)for n ≥ 2 and 1 ≤ k ≤ n−1,

and[n0

]F=[nn

]F= 1 for all n ≥ 0.

The Fibonomial coefficients and the Lucasu-nomial coefficients wereintroduced in 1878 byE. Lucas [87,§9], and later they have been studiedby several authors (see [50], [59], [60], [139], [69] and [68]).

The triangle of Fibonomial coefficients is given as Sloane’ssequenceA010048 in [124]. It is known (see, e.g., [59, the equality (D), page 386])that

[n

k

]

F

= Fk+1

[n− 1

k

]

F

+ Fn−k−1

[n− 1

k − 1

]

F

, for 0 ≤ k ≤ n− 1,

whence by induction immediately follows that the all Fibonomial coeffi-cients are integers.

WhenA = q+1 andB = q related to the sequence defined by (77), whereq is an integer such that|q| > 1,

[nk

]u, then it coincides with theGaussian

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 30

q-nomial coefficient[nk

]q

becauseuj = (qj − 1)/(q − 1) for j = 1, 2, . . .,and hence, [

n

k

]

q

=(qn − 1)(qn−1 − 1) · · · (qn−k+1 − 1)

(qk − 1)(qk−1 − 1) · · · (q − 1).

The numbers[nk

]q

were introduced in 1808 by Gauss [46,§5]. It is wellknown that these numbers satisfy the recursion formula

[n

k

]

q

= qk[n− 1

k

]

q

+

[n− 1

k − 1

]

q

, for 0 ≤ k ≤ n− 1.

The triangles of Gaussianq-nomial coefficients forq = −2, 2, 3, 4, 5, 6, 7, 8, 9are given as Sloane’s sequences A015109, A022166, A022167,A022168,A022169, A022170, A022171, A022172 and A022173 in [124], respec-tively.

It is easy to see that if0 ≤ m ≤ n, then

limq→1

[n

m

]

q

=

(n

m

),

[n

m

]

q

=

[n

n−m

]

q

(symmetry)

and [n

m

]

q

=

[n− 1

m− 1

]

q

+ qm[n− 1

m

]

q

,

whence easily follows by induction that ifq is any positive integer, then[nm

]q

are also integers for alln andm. �

Remark26. An analogy to the Lucasu-nomial coefficients[nk

]u

wasobtained in 1995 by W.A. Kimball and W.A. Webb [77] and in 1998by B.Wilson [140] in some special cases, and in 2001 by H. Hu and Z.-W. Sun[69] for the general case (see Subsection 5.2). �

It is known (see, e.g., [84], [139]) that the generalized base for the Fi-bonacci sequence is

P = {r0, r1, r2, r3, r4, . . . , } = {1, 3, 6, 6, 12, . . . , }in the sense that any positive integern can be uniquely expressed as

n = (nsns−1 . . . n1n0)P := n0 + n1r1 + · · ·+ ns−1rs−1 + nsrs,

where0 ≤ ni < ri+1/ri for eachi = 0, 1, . . . , s− 1.Under the above notations, in 1994 D.L. Wells [139, Theorem 2] proved

that

(79)

[n

k

]

F

≡[n0

k0

]

F

·∏

i≥1

(ni

ki

)(mod 2).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 31

In 1988 M. Sved [131] establihed that the geometry of the binomial ar-rays of Pascal’s triangle modulop gives a simple interpretation of Lucas’theorem. Moreover, as noticed in [131, p. 58], this interpretation can beextended to arrays of other combinatorial functions; in particular, Lucas’theorem can be generalized to the Gaussianq-nomial coefficients as fol-lows. Let p be a prime,q > 1 a positive integer not divisible byp, andlet a 6= 1 be the minimal exponent for whichqa ≡ 1 (mod p); then by Fer-mat little theorem it follows thata | (p − 1). Further, if n = Na + n0,m =Ma +m0 with 0 ≤ n0 < a and0 ≤ m0 < a, then[131, p. 60]

(80)

[n

m

]

q

≡(N

M

)[n0

m0

]

q

(mod p).

Remark27. In the same area of research A. Bes [16] generalized Lucas’theorem. This accomplishment obviously serves to improve the security ofcryptographic applications modulo prime powers [16]. �

Definition. For a positive integerd, therank of apparitionr = r(d) withrespect to the integer sequence(an)n≥0 is the least indexn for which ddividesan, that is,r(d) = min{n ∈ N : d | an} (if d does not divide anyan, thenr(d) = ∞). �

Remark28. Leta = (an)n≥0 be an integer sequence. In order to guar-antee that the alla-nomial coefficients

[nk

]a= 0 are integers, it is usually

required that the sequencea = (an)n≥0 beregularly divisible, that is,pi | ajif and only if r(pi) | j for all i ≥ 1, j ≥ 1, and all primesp. Herer(pi)denotes the rank of apparition ogpi as defined above. The principal classof sequences which are known to be regularly divisible are the Lucas se-quences given by (77) for whichgcd(A,B) = 1 (see [63]). �

In 2000 J.M. Holte [61, Theorem 1] proved the following result: Let pbe a prime and letm and n be nonnegative integers. Letr be the rankof apparition ofp with respect to the Lucas sequenceu = (un), let τ bethe period of(un) modulop, and lett = τ/r (t is necessarily a positiveinteger). Furthermore, fori, j ≥ 0 and for 0 ≤ k, l < r, let Ai,j(k, l)denote the solution of the modulop recurrence relation

Ai,j(k, l) ≡ uir+k+1Ai,j(k, l − 1) + bujr+l−1Ai,j(k − 1, l) (mod p),

and letHi,j(k, l) = urijr+1Ai,j(k, l). Setn0 = n(mod r), m0 = m(mod r),n′ = n + r,m′ = m+ r, n′′ = n′(mod t), andm′′ = m′(mod t). Then

(81)

[m+ n

n

]

u

≡(m′ + n′

n′

)Hm′′,n′′(m0, n0) (mod p).

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 32

Using the above result, with the same notations as above, Holte [61, The-orem 3] also proved the following result:Let (un) be the Lucas sequencedefined by(77), let p be a prime such thatB is not divisible byp. Setλ = max{0, m′′ + n′′ − (p − 1)}, n∗ = n(mod t) andm∗ = m(mod t).Then(82)[

m+ n

n

]

u

≡(m′ + n′

n′

)(m′′ + n′′ + λt,

n′′ + λt

)−1[m∗

n∗ + λτ

]

u

(mod p).

Thus, except whens = p− 1 andm′′ + n′′ ≥ p, then

(83)

[m+ n

n

]

u

≡(m′ + n′

n′

)(m′′ + n′′ + λt,

n′′

)−1[m∗

n∗

]

u

(mod p).

Holte [61, Section 7] noticed that by means of a bit of translation, thecongruence (82) may be transformed into the following result obtained in1992 by D. Wells [137] (also see [138]):Let N = n + m, and corre-spondingly,N0 = N(mod r), N ′ = ⌊N/r⌋, andN ′′ = N ′(mod s). LetN ′ =

∑lj=0Njp

j andm′ =∑l

j=0mjpj be thep-adic expansions ofN ′ and

m′. If p is a prime such thatB is not divisible byp, then under the samedefinitions ofB andt as above, forN ′′ ≥ m′′,

(84)

[N

m

]

u

≡(N ′′

m′′

)−1 l∏

j=0

(Nj

mj

)[Nr +N0

m′′r +m0

]

u

(mod p),

and forN ′′ < m′′,(85)[N

m

]

u

≡{ (

s+N ′′

m′′

)−1∏lj=0

(Nj

mj

)[t+N ′′r+N0

m′′r+m0

]u

(mod p) if s < p− 1(s

m′′

)−1∏lj=0

(Nj

mj

)[(N ′′+1)t+N ′′r+N0

m′′r+m0

]u

(mod p) if s = p− 1.

Remark29. In 2002 E.R. Tou [135, Theorem 4] generalized the congru-ence (82) modulo product of a finite number of distinct primes. �

5.2. Lucas type congruences for some classes of Lucasu-nomial coeffi-cients. In 2001 H. Hu and Z.-W. Sun [69, Theorem] proved the followingresult for the Lucasu-nomial coefficients:Let u = (un)n≥0 be a Lucassequence defined by(77). Suppose thatgcd(A,B) = 1, andA 6= ±1 orB 6= ±1. Thenuk 6= 0 for everyk ≥ 1. Let q be a positive integer, letm and n be nonnegative integers, and letR(q) = {0, 1, . . . , q − 1}. Ifs, t ∈ R(q) then

(86)

[mq + s

nq + t

]

u

≡(m

n

)[s

t

]

u

u(nq+t)(m−n)+n(s−t)q+1 (mod wq),

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 33

wherewq is the largest divisor ofuq relatively prime tou1, . . . , uq−1. If q orm(n+ t) + n(s+ 1) is even, then(87)[mq + s

nq + t

]

u

≡(m

n

)[s

t

]

u

(−1)(mt−ns)(q−1)Bq2((nq+t)(m−n)+n(s−t)) (mod wq).

Remark30. ([69, Remark 1]) WhenA = 2 andB = 1, we haveuk = kfor each nonnegative integerk, and if in addition we assume thatq = p is aprime, thenwp = p, and hence the congruence (86) becomes

(mp + s

np+ t

)≡(m

n

)(s

t

)(mod p),

which is in fact, Lucas’ theorem. �

In 2002 H. Hu [68, p. 291, Theorem] proved the following result: Letq be a positive integer, and letm andn be even nonnegative integers withn ≤ m. Lets andt be nonnegative integers such thatt ≤ s < q, and letv∗qbe the largest divisor ofvq relatively prime tov0, . . . , vq−1. Then(88)(

m/2

n/2

)[mq + s

nq + t

]

u

≡(m

n

)[s

t

]

u

(−Bq)m−n

2(nq+t)+n

2(s−t) (mod v∗q ).

Lucas type congruences modulop2 andp3 (p is a prime> 3) for Lucasu-nomial coefficients and Fibonomial coefficients are established in [76],[77] and [120]. Namely, in 1993 W.A. Kimball and W.A. Webb [76] (alsosee [120, p. 1029]) proved the following two results:Letp be an odd primeand letm andn be nonnegative integers. Suppose thatτ is the period of theFibonacci sequence(Fn)n≥0 modulop, r is the rank of apparition ofp (thatis, r is the least indexk for whichp dividesFk), andt = τ/r is an integer.In [134] it is shown thatt ∈ {1, 2, 4}. The numberε is defined as follows:ε = 1 if τ = r; ε = −1 if τ = 2r; and ε2 ≡ −1(mod p2) if τ = 4r; in thiscasep ≡ 1(mod 4). Then

(89)

[mτ

]

F

≡(mt

nt

)(mod p2)

and

(90)

[mr

nr

]

F

≡ ε(m−n)nr

[m

n

]

F

(mod p2).

In 1995 Kimball and Webb [77, Theorems 1 and 3] proved the followingresults:Let (un)n≥0 and(vn)n≥0 be the sequences defined by(77)and(78),respectively, whereA andB are nonzero integers such thatgcd(A,B) = 1.Let p be an odd prime, letτ be the period of the sequence(un)n≥0 modulo

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 34

p, and letr be the rank of apparition ofp. Then for all nonnegative integersm andn such thatn ≤ m there holds

(91)

[mr

nr

]

u

≡(vr2

)(m−n)nr(m

n

)(mod p2)

and

(92)

[mτ

]

u

≡(1 +

1

2τ(m− n)n((−B)τ − 1)

)(mt

nt

)(mod p2).

As a consequence of the congruence (91), it is proved in [77, Corollary 2]that

(93)

[mτ

]

u

≡(1 + τ(m− n)n

((vr2

)t− 1

))(mt

nt

)(mod p2).

Moreover, the congruence (92) immediately implies [77, Corollary 4] thatif B = ±1, then

(94)

[mτ

]

u

≡(mt

nt

)(mod p2).

Kimball and Webb [77, Theorem 5] also proved the following congru-ences for the Gaussianq-nomial coefficients:

[mr

nr

]

q

≡(qr + 1

2

)(m−n)nr (m

n

)(mod p2)

≡(1 +

1

2r(m− n)n(qr − 1)

)(m

n

)(mod p2),

(95)

wherep is a prime,q is anyp-integral rational number such thatq2 − q isnot divisible byp, andr is the rank of apparition ofp.

In 1998 B. Wilson [140] proved the following result:Let p be a primesuch thatp 6= 2, 5, and letr be the rank of apparition ofp with respect tothe Fibonacci sequence(Fn)n≥0. Then for any nonnegative integersm,n, sandl such that0 ≤ s, l < r

(96)

[mr

nr

]

F

≡[m

n

]

F

F(m−n)nrr+1 (mod p)

and

(97)

[mr + s

nr + l

]

F

≡(m

n

)[s

l

]

F

F(nr+l)(m−n)+n(s−l)r+1 (mod p).

In 2007 L.-L. Shi [120] proved another congruence modulop2 (wherep > 3 is a prime) for the Lucasu-nomial coefficients. Namely, in [120,Theorem 2] it is proved the following result:Let (un)n≥0 be the Lucassequence defined by(77), whereA andB are nonzero integers such that

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 35

gcd(A,B) = 1, andA 6= ±1 or B 6= 1. Letp > 3 be a prime not dividingB. If r is the rank of apparition ofp with respect to(un)n≥0, then for anynonnegative integersm,n, s andt such that0 ≤ s, l < r, we have(98)[mr + s

nr + l

]

u

(−1)l−s−1B−(l−s2 )u(m−n)ru

−1l−s

×u(m−n)(l−1)−n(l−s)r+1

[mrnr

]u

([ls

]u

)−1(mod p2) if s < l

uml+ns−2nlr+1

Sm,s

Sn,lSm−n,s−l

[mrnr

]u

[sl

]u

(mod p2) if s ≥ l,

whereSk,i = 1− (kBur)/ur+1

∑ij=1(uj−1/uj).

If ∆ := A2 − 4B is not divisible byp, then[mrnr

]u

in (98)can be replacedby (vr/2)(m−n)nr

(mn

).

In 1995 Kimball and Webb [78, Theorem] and in 2007 L.-L. Shi [120]considered thegeneralized Lucasu-nomial coefficientsand thegeneralizedFibonomial coefficientsdefined as follows. If(un)n≥0 is the Lucas sequencedefined by (77) such thatA 6= ±1 or B 6= 1, and let(Fn)n≥0 be the Fi-bonacci sequence. For any positive integerj we set

[n]ju =n∏

k=1

ukj and [n]jF =n∏

k=1

Fkj,

for n = 0, 1, 2, . . ., and regard an empty product as value 1.Then forn, k = 0, 1, 2, . . . the generalized Lucasu-nomial coefficient[

nk

]ju

and thegeneralized Fibonomial coefficient[nk

]jF

are defined as follows:[n

k

]j

u

=

{[n]ju

[k]ju[n−k]juif 0 ≤ k ≤ n

0 otherwise,

[n

k

]j

F

=

{[n]jF

[k]jF [n−k]juif 0 ≤ k ≤ n

0 otherwise.

where(uij/uj)i≥0 is also a Lucas sequence.In 1995 Kimball and Webb [78, Theorem] extended the congruence (90)

by showing thatif the rankr of apparition ofp is p + 1 or p − 1, then forany primep > 3 and anym ≥ n ≥ 0,

(99)

[mr

nr

]

F

≡ (∓)(m−n)n

[m

n

]r

F

(mod p3), respectively.

In 2007 Shi [120] proved the congruence modulop3 (wherep > 3 is aprime) for the generalized Lucasu-nomial coefficients. Namely, in [120,Theorem 1] it is proved the following result:LetA andB be nonzero inte-gers such thatgcd(A,B) = 1, andA 6= ±1 or B 6= 1. Letp > 3 be a primenot dividingB. If the rankr of apparition ofp is p+ 1 or p− 1 (and hence

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 36

r = p−(

A2−4Bp

)), where

(·p

)denotes the Legendre symobol, then for any

nonnegative integersm andn we have

(100)

[mr

nr

]

u

≡ (−1)(m−n)nB(m−n)n(r2)[m

n

]r

u

(mod p3).

Remark31. In the caseA = −B = 1 the congruence (100) yields thecongruence (99) of Kimball and Webb [78]. �

In 1965 G. Olive [104] (also see [105, Lemma 2.1]) proved the followingresult: Suppose thatd is a positive integer anda, b, h, l are integers suchthat0 ≤ b, l ≤ d− 1. Then

(101)

[ad+ b

hd+ l

]

q

≡(a

h

)[b

l

]

q

(mod Φd(q)),

whereΦd(q) is thedth cyclotomic polynomial.

Remark32. As noticed in [119, Chapter 5, p. 506], the congruence(101) perhaps was known to Gauss and it is rediscovered in 1982 by J.Desarmenien [32] and V. Strehl [128] whose proof uses combinatorial ar-guments. �

Remark33. Another differentq-analogue of the congruence (101) wasestablished in 1967 by R.D. Fray [42]. �

Remark34. Applying Lucas’ theorem, in 2006 S.-P. Eu, S.-C. Liu and Y.-N. Yeh [37] established the congruences of several combinatorial numbers,including Delannoy numbers and a class of Apery-like numbers, the num-bers of noncrossing connected graphs (Sloane’s sequence A007297), thenumbers of total edges of all noncrossing connected graphs on n vertices(Sloane’s sequence A045741), etc. �

6. SOME APPLICATIONS OFLUCAS’ THEOREM

Even today, Lucas’ theorem is being studied widely, and has both ex-tended and generalized, particularly in the area of divisibility of binomialcoefficients. Numerous results on divisibility of binomialand multinomialcoefficients by primes and prime powers and related historical notes aregiven in 1980 by D. Singmaster [122]. Furthermore, Lucas’ theorem hasnumerous applications in Number Theory, Combinatorics, Cryptographyand Probability. We also point out that this theorem has become ubiquitousin the Theory of cellular automata.

6.1. Lucas’ theorem and the Pascal’s triangle.Let ak(n) be the numberof integers0 ≤ m ≤ n such that

(nm

)6≡ 0(modk), that is,ak(n) is the

number of nonzero entries on rown of Pascal’s triangle modulok. Let|n|w be the number of occurrences of the wordw in nsns−1 · · ·n0, where

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 37

n =∑s

i=0 niki is the base-k representation ofn. In 1899 J.W.L. Glaisher

[48, §14] initiated the study of counting entries on rown of Pascal’s trianglemodulok by using Lucas’ theorem to determinea2(n) = 2|n|1. The proofis simple (cf. [114, p. 1]): In order that

(nm

)be odd, each term

(ni

mi

)in

the product must be 1, so ifni = 0 thenmi = 0 and if ni = 1 thenmi

can be either 0 or 1. It was the first result on a thorny path of solutionof this difficult problem. However, this topic was forgottenfor almost ahalf-century.

In 1947 N.J. Fine [39] generalized Glaisher’s result to an arbitrary prime.Fine’s result follows from Lucas’ theorem in the same way:Let p be aprime, and letn be a nonnegative integer. The number of nonzero entrieson rown =

∑si=0 nip

i of Pascal’s triangle modulop is (cf. [114, p. 2])

(102) ap(n) =

s∏

i=0

(ni + 1).

Namely, the formula (102) immediately follows from the factthat by Lucas’theorem, the binomial coefficient

(nm

)with m =

∑si=0mip

i is not divisibleby a primep if and only if 0 ≤ mi ≤ ni for all i = 0, 1, . . . , s.

Remark.35. If p = 2, then the formula (102) presents the number of oddentries on rown =

∑si=0 ni2

i of Pascal’s triangle. Notice that the parityof binomial coefficients has played an important role in a paper from 1984of J.P. Jones and Y.V. Matijasevic [73] in connection with Hilbert’s tenthproblem, Godel’s undecidability proposition and computational complexity.They base their Lemma on the Lucas’ theorem given by the congruence (1)with p = 2 (cf. [74, Lemmas 3.9 and 3.10]). �

As noticed in [114], one may generalize Glaisher’s result ina differentdirection, namely to ask for the numberak,r of integers0 ≤ m ≤ n suchthat

(nm

)≡ r(modk). In 2011 E. Rowland [114, Section 2, Theorem 1]

generalized Fine’s result to prime powers, obtaining a formula for the sumapα(n) =

∑pα−1,rr=1 (n). Notice that in 1978 E. Hexel and H. Sachs [58,§5]

determined a formula forap,ri(n) in terms of(p−1)th roots of unity, wherer is a primitive root modulop. For some related results see also [5], [28],[44], [51] and [114, Theorem 2]).

The previous considerations can be genearlized as follows.Let p be aprime. For nonnegative integersn andk consider the set

A(p)n,k = {j ∈ {0, 1, . . . , n} : pk‖

(n

j

)},

wherepk‖(nj

)denotes thatpk |

(nj

)and

(nj

)6≡ 0(mod pk+1). In particular,

A(p)n,0 is a set of nonzero entries on rown of Pascal’s triangle modulok.

Therefore, under the previous notation, for a primep we haveap(n) =

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 38

|A(p)n,0| (|S| denotes the cardinality of a setS), Notice that|A(p)

n,0| can beevaluated by Fine’s formula (102). In 1967 L. Carlitz [23] solved a difficultproblem for evaluation of|A(p)

n,1|. In 1971 F.T. Howard [64], discovered the

formula for|A(2)n,k| for arbitraryk. In 1973 F.T. Howard [65] found a solution

for |A(p)n,2|.

Further related results are given in [52], and in 1997 by J.G.Huard, B.K.Spearman and K.S. Williams [70]. Letn be a nonnegative integer. Thenthrow of Pascal’s triangle consists of the followingn+1 binomial coefficients:

(n

0

),

(n

1

),

(n

2

), . . . ,

(n

n

).

We denote byNn(t,m) the number of those binomial coefficients whichare congruent tot modulom, wheret andm ≥ 1 are integers such that0 ≤ t ≤ m − 1. Let p be a prime, and letn be a positive integer with thep-adic expansionn =

∑ki=0 nip

i. We denote the number ofr’s occuringamongn0, n1, . . . , nk by lr (r = 0, 1, . . . , p− 1). Setω = e2πi/(p−1) and letg denote a primitive root modulop. Denote byindgt the index of the integert 6≡ 0(mod p) with respect tog; that is,indgt is the unique integerj suchthat t ≡ gj(mod p). In 1978 E. Hexel and H. Sachs [58, Theorem 3] haveshown thatfor t = 1, 2, . . . , p− 1,

(103) Nn(t, p) =1

p− 1

p−2∑

s=0

ω−sindgt

p−1∏

r=1

B(r, s)lr ,

where

B(r, s) =

r∑

j=0

ωsindg(rj).

By using the formula (103), in 1997 J.G. Huard, B.K. Spearmanand K.S.Williams proved the analogous formula forNn(tp, p

2)with t = 1, 2, . . . , p−1 [70, Theorem 1.1]. They proved thatfor t = 1, 2, . . . , p− 1,

Nn(tp, p2) =

1

p− 1

p−2∑

i=0

p−1∑

j=1

lij

p−2∑

s=0

ω−s(indgt+indg(i+1)−indgj)

×B(p− 2− i,−s)B(j − 1, s)

p−1∏

r=1

B(r, s)lr−δ(r−i)−δ(r−j),

(104)

where

δ(x) =

{1 if x = 00 if x 6= 0,

andlij denotes the number of occurences of the pairij in the stringn0n1 . . . nk.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 39

Let p be a prime, and letk be a positive integer. LetA(k, p) be the matrixwith entries

(ij

)p:=(ij

)(mod p), 0 ≤ i < pk, 0 ≤ j < pk (actually,

(ij

)p

is

the remainder of the division of(ij

)by p). By using the Lucas property of

the matrixA(k, p) given by (54), in 1994 M. Razpet [111, p. 378]provedthat the number of all zero entries of the matrixA(k, p) is equal top2n −(p+12

)k, and hence, the number of all nonzero entries of the matrixA(k, p)

is equal to(p+12

)k.

Let p be a prime, and letn be a positive integer. For an integerr suchthat0 ≤ r ≤ p − 1, let br(n) be the number of binomial coefficients

(ij

)

with 0 ≤ j ≤ i < n such that(ij

)≡ r(modp). In 1957 J.B. Roberts

[113] established systems of simultaneous linear difference equations withconstant coefficients whose solutions would yield the quantities br(n) ex-plicitly. Namely, if 0 ≤ c ≤ p − 1, 1 ≤ t ≤ pk, k > 0, and if q is thereciprocal ofq ∈ {1, 3, . . . , p−1} modulop (i.e.,qq ≡ 1(mod p)), then by[113, Theorem 1],

(105) br(cpk + t) = br(cp

k) +

p−1∑

q=1

(brq(c+ 1)− brq(c))bq(t).

Furthermore, ifb(n) =∑p−1

r=1 br(n) andn =∑k

i=0 nipi with0 ≤ ni ≤ p−1

for all i = 0, 1, . . . , k, then by[113, Corollary 4],

(106) b(n) =1

2

k∑

i=0

ni((ni + 1) · · · (nk + 1))

(1

2p(p+ 1)

)i

.

By using Lucas’ theorem, in 1992 R. Garfield and H.S. Wilf [44,The-orem] proved the following result:Let p be a prime, leta be a primitiveroot modulop, and letn be a nonnegative integer with thep-adic expan-sionn =

∑si=0 nip

i. Denote bylj(n) the number ofj’s occuring amongn0, n1, . . . , ns (j = 0, 1, . . . , p− 1). Further, for eachi ∈ {0, 1, . . . , p− 2}let ri(n) be the number of integersk with 0 ≤ k ≤ n, for which

(nk

)≡

ai(mod p), and letRn(X) =∑p−2

i=0 ri(n)Xi be their generating function.

Then

(107) Rn(X) ≡p−1∏

j=1

Rj(X)lj(n) (mod Xp−1 − 1).

In 1990 R. Bollinger and C. Burchard [17] considered the extended pas-cal’s triangles which arise, by analogy with the ordinary Pascal’s triangle asthe (left-justified) arrays of the coefficients in the expansion (1 + x+ x2 +· · · + xk−1)n. That is, the arrayTk has in rown, columnm, the number

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 40

Ck(n,m) defined fork, n,m ≥ 0 by the expansion

(1 + x+ x2 + · · ·+ xk−1)n =

(k−1)n∑

m=0

Ck(n,m)xm,

It is nociced in [17, the property d) on page 199] that

Ck(n,m) =∑

j

(−1)j(n

j

)(n− 1 +m− kj

n− 1

),

and hence,C2(n,m) =(nm

). Accordingly,T2 is the Pascal’s triangle.

R. Bollinger and C. Burchard [17, Theorem 1] applied Lucas’ theoremto the Pascal’s triangle, proving thatif p is a prime, and ifn = n0 + n1p +· · ·+ nsp

s andm = m0 +m1p + · · ·+mspl are thep-adic expansions of

n andm, then

(108) Ck(n,m) ≡∑

r0,...,rs

s∏

i=0

Ck(ni, ri) (mod p),

where the sum is taken over all(s + 1)-tuples(r0, r1, . . . , rs) such thati)m = r0 + r1p + · · · + rsp

s and ii) 0 ≤ ri ≤ (k − 1)ni for eachi =0, 1, . . . , s; if m is not representable in this form, then certainlyCk(n,m) ≡0 (mod p).

6.2. Another applications of Lucas’s theorem. By using Kummer’s the-orem and Lucas’ theorem, in 2007 K. Dilcher [34, Theorem 2] derived analternating sum analog to a special case to an 1876 congruence of Hermite[57] (also see [36, Chapter IX, p. 271]) as follows.Let p be an odd primeand letq be a positive integer. Then(109)⌊q/2⌋∑

j=0

(q(p− 1)

2j(p− 1)

)≡

1 (mod p) if q is odd;2 (mod p) if q is even and q 6≡ 0(mod p+ 1);32

(mod p) if p+ 1 | q.By using Lucas’ theorem, in 2009 the author of this article proved the

following result [92, Theorem].If d, q > 1 are integers such that

(110)

(nd

md

)≡(n

m

)(mod q)

for every pair of integersn ≥ m ≥ 0, thend andq are powers of the sameprimep.

Remark36. Observe that the above result may be considered as a partialconverse theorem of the congruence (5) of Subsection 2.1. �

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 41

In 2010 M.P. Saikia and J. Vogrinc [116, Theorem 2.1] (see also [81,Theorem 1.2 and its proof]) proved thata positive integerp > 1 is a primeif and only if

(111)

(n

p

)≡⌊n

p

⌋(mod p)

for every nonnegative integern.By using Lucas’ theorem, in 2013 the author of this article [99, Theorem

1.1] generalized Babbage’s criterion for primality given in 1819 by Babbage[9] (also see [52, Section 4]). Lucas’ theorem is also applied in a recentauthor’s note [101, Theorem 1] in order to prove the following result: Ifn > 1 andq > 1 are integers such that

(n− 1

k

)≡ (−1)k (mod q)

for every integerk ∈ {0, 1, . . . , n− 1}, thenq is a prime andn is a powerof q.

Definition (see, e.g., [2]). Letp be a prime. We say that the sequenceof rational numbers(an)n≥0 (an)n≥0 has thep-Lucas property(or that thesequence(an)n≥0 is p-Lucas) if the denominators of all thean’s are notdivisible byp, and if for alln ≥ 0 and for allj ∈ {0, 1, . . . , p− 1} it holds

(112) apn+j ≡ anaj (mod p). �

Clearly, the sequence of rational numbers(an)n≥0 has thep-Lucas propertyif and only if

(113) an ≡s∏

i=0

ani(mod p),

for every positive integern with thep-adic expansionn = n0+n1p+ · · ·+nsp

s such that0 ≤ ni ≤ p − 1 for all i = 0, 1, . . . , s. Furthermore, theinteger sequence(an)n≥0 has the Lucas property if and only if(an)n≥0 hasthep-Lucas property for every primep.

In what follows, we will consider sequences(an)n≥0 having thep-Lucasproperty for infinitely many primesp. As noticed in [2, Remarks 1], such asequence is either 0 or it satisfiesa0 = 1. �

For a positive integert considerthe formal power series∞∑

n=0

(2n

n

)t

Xn.

It is known that the above formal power series is transendental overQ(X)whent ≥ 2. This is due in 1980 to Stanley [125], and independently in 1987to Flajolet [40] and in 1989 to C.F. Woodcock and H. Sharif [143]. While

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 42

Stanley and Flajolet used analytic methods and studied the asymptotics ofthe coefficients of this series, Woodcock and Sharif gave a purely algebraicproof. Their basic idea is to reduce this series modulo a primep, and to usethep-Lucas property for central binomial coefficients: ifn =

∑si=0 ni is the

basep expansion of a positive integern, then ([89]; cf. (58) of Subsection4.1)

(114)

(2n

n

)≡

s∏

i=0

(2ni

ni

)(mod p).

Namely, a proof of Woodcock and Sharif [143] is based on the followingcongruence which follows from Lucas’ theorem:

F p−1t (X) ≡

(p−1)/2∑

i=0

(2i

i

)X i

−1

(mod p).

In 1998 J.-P. Allouche, D. Gouyou-Beauchamps and G. Skordev[2] gener-alized the method of Woodcock and Sharif to characterize allformal powerseries that have thep-Lucas property for “many” primesp, and that are fur-thermore algebraic overQ(X). Namely, they proved the following result[2, Theorem 1]:Let s be an integer≥ 2. Defines′ = s if s is even, ands′ = 2s if s is odd. LetF (X) =

∑∞n=0 anX

n be a nonzero formal powerseries with coefficients inQ. Then the following conditions are equivalent:

(i) The sequence(an)n≥0 has thep-Lucas property for all large primesp such thatp ≡ 1(mod s), and the formal power seriesF (X) isalgebraic overQ(X).

(ii) There exists a polynomialP (X) in Q[X ] of degree at mosts′, withP (0) = 1, such thatF (X) = (P (X))−1/s′.

If s is odd, and if the numbers′ is replaced bys in the statement(ii), westill have(ii) implies(i), but the converse is not necessarily true.

Furthermore, when the numbers is equal to 2, in 1999 Allouche [1, The-orem 6.4] proved the following result (cf. [2, Theorem 2]):Let (an)n≥0 bea nonzero sequence of rational numbers. Then the following assertions areequivalent.

(i) The sequence(an)n≥0 has thep-Lucas property for all large primesp, and the seriesF (X) =

∑∞n=0 anX

n is algebraic overQ(X).(ii) For all large primesp the sequence(an)n≥0 has thep-Lucas prop-

erty, and the degreedp of the series∑∞

n=0(an(mod p))Xn (that isnecessarily algebraic overFp(X) from the p-Lucas property) isbounded independently ofp.

(iii) There exists a polynomialP (X) in Q[X ] of degree at most2, withP (0) = 1, such thatF (X) =

∑∞n=0 anX

n = (P (X))−1/2.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 43

Remark37. In 2013E. Delaygue [31, Subsection 1.2] considered thenotion ofp-Lucas property to aZp-valued familyA = (A(n))

n∈Nd, wherep is a prime,Zp is the ring ofp-adic integers andd is a positive integer.We say thatA satisfies thep-Lucas property if and only if, for allv ∈{0, 1, . . . , p− 1}d and alln ∈ Nd, we have

A(v + np) ≡ A(v)A(n) (mod pZp).

Delaygue [31, Theorem 3] established an effective criterion for a sequenceof factorial ratios to satisfy thep-Lucas property for almost all primesp. �

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APPENDIX

List of references and related Lucas type congruencesfrom this article (arranged by year of publication)

[9, 1819] Charles Babbage (also see [52, Introduction] or [36, page 271]) - (19),p. 10.

[142, 1862] Joseph Wolstenholme - (20), p. 10.[7, 1869] H. Anton (also see [36, p. 271]) - (2), p. 5)[86, 1878], [87, 1878; Section XXI, pp. 229–230],E. Lucas (Lucas’ theorem) -

the congruences (1) and (3), p. 5.[49, 1900; p. 323] J.W.L. Glaisher (also see [103, the congruence 7.1.5] and

[93, Section 6]) - (21), p. 10.[19, 1949] W. Ljunggren (also see [10, Theorem 4], [52], [126, Problem 1.6 (d)]

and [123]) - (22), p. 11.[19, 1952] E. Jacobsthal (also see [52]) - (23), p. 11.[22, 1955] L. Carlitz - (70), p. 25.[26, 1956] L.E. Clarke and [108, 1957], P.A. Piza - (7), p. 8.[104, 1965] G. Olive (also see [119, Chapter 5, p. 506], [32],[128] [105, Lemma

2.1]) - (101), p. 36.[47, 1982; Theorem 1] I. Gessel - (52), p. 19.[66, 1987; Theorem 1] F.T. Howard - (69), p. 25.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 50

[66, 1987; p. 306, Corollary and Theorem 2] F.T. Howard - (71)and (72), p. 25.[88, 1988; Theorem 2], R.A. Macleod - (43), p. 16.[131, 1988; p. 61, Theorem] M. Sved - (64), p. 23.[131, 1988; p. 60, Theorem] M. Sved - (80), p. 31.[10, 1990; Theorem 3] D.F. Bailey - (31), p. 12.[10, 1990; Theorem 5] D.F. Bailey - (32), p. 13.[17, 1990; Theorem 1] R. Bollinger and C. Burchard - (108), p.40.[11, 1991; Theorem 4] D.F. Bailey - (26), p. 11.[11, 1991; Theorem 5] D.F. Bailey - (27), p. 12.[29, 1991; Theorem 3] K. Davis and W. Webb (also see [85, p. 88,Theorem

5.1.2] and [15, p. 34, congruence (2.2)]) - (37), p. 14.[12, 1992; Theorem 5] D.F. Bailey (also see [98, Corollary 1.2]) - (28) and (29),

p. 12.[51, 1992; Proposition 2] A. Granville - (34), p. 13.[89, 1992] R.J. McIntosh - (114), p. 42.[137, 1992; Theorem 2] (also see [138] and [61, Section 7]) D.L. Wells - (84)

and (85), p. 32.[30, 1993; Theorem 3] K. Davis and W. Webb (also see [51, Proposition 2]) -

(40), p. 15.[30, 1993; Corollary 1] K. Davis and W. Webb - (41), p. 15.[30, 1993; Corollary 1] K. Davis and W. Webb (also see [97, Theorem]) - (42),

p. 15.[76, 1993] W.A. Kimball and W.A. Webb (also see [120, p. 1029]) - (89) and

(90), p. 33.[107, 1993; Proposition 2.1] R. Peele, A.J. Radcliffe and H.S. Wilf - (68), p. 24.[60, 1994; p. 60] J.M. Holte (also see [61, 1994; p. 227]) - (14) and (15), p. 9.[111, 1994; p. 378] M. Razpet - (54), p. 20.[139, 1994; Theorem 2] D.L. Wells - (79), p. 30.[52, 1995; Section 6, the congruence (24)] A. Granville - (16), p. 9.[52, 1995; Theorem 1] A. Granville - (33), p. 13.[77, 1995; Theorems 1 and 3] W.A. Kimball and W.A. Webb - (91) and (92), p.

34.[77, 1995; Corollaries 2 and 4] W.A. Kimball and W.A. Webb - (93) and (94),

p. 34.[77, 1995; Theorem 5] W.A. Kimball and W.A. Webb - (95), p. 34.[78, 1995; Theorem] W.A. Kimball and W.A. Webb - (99), p. 35.[21, 1998; Lemma 4] N.J. Calkin - (56), p. 20.[140, 1998] B. Wilson - (96) and (97), p. 34.[1, 1999; Proposition 7.1] J.-P. Allouche - (51), p. 18.[61, 2000; Theorem 1] J.M. Holte - (81), p. 31.[61, 2000; Theorem 3] J.M. Holte - (82) and (83), p. 32.[118, 2000; Proposition 3.1] R. Sanchez-Peregrino - (65) and (66), pp. 23–24.[118, 2000; Proposition 4.1] R. Sanchez-Peregrino - (67),p. 24.[18, 2001; Theorem 2.2] J. Boulanger and J.-L. Chabert - (76), p. 27.[69, 2001; Theorem] H. Hu and Z.-W. Sun - (86) and (87), pp. 32–33.

LUCAS’ THEOREM: ITS GENERALIZATIONS, EXTENSIONS... 51

[68, 2002; p. 291, Theorem] H. Hu - (88), p. 33.[112, 2002; Theorem 1] M. Razpet - (62), p. 22.[14, 2003; Theorem 5] D. Berend and N. Kriger - (55), p. 20.[106, 2004; Theorem 1] H. Pan - (63), p. 23.[38, 2005; Theorem 3] T.J. Evans - (17), p. 9.[33, 2006; Theorem 4.7] E. Deutsch and B.E. Sagan - (57), p. 21.[33, 2006; Theorem 4.4] E. Deutsch and B.E. Sagan - (58), p. 21.[120, 2007; Theorem 2] L.-L. Shi - (98), p. 35.[120, 2007; Theorem 1] L.-L. Shi - (100), p. 36.[129, 2007; Theorem 1.7] Z.-W. Sun and D.M. Davis - (75), p. 26.[56, 2008; the congruence (1) of Corollary on page 490] C. Helou and G. Terja-

nian (also see [27, Section 11.6, Corollary 11.6.22, p. 381]) - (24), p. 11.[72, 2008; (ii) of Lemma 2] Y. Jin, Z-J. Lu and A.L. Schmidt - (53), p. 19.[130, 2008; Theorem 1.1] Z.-W. Sun and D. Wan - (74), p. 26.[25, 2009; Theorem 2.2] M. Chamberland and K. Dilcher - (59),p. 22.[25, 2009; Corollaries 2.1 and 2.2] M. Chamberland and K. Dilcher - (60) and

(61), p. 22.[103, 2011; the congruences 7.3.1–7.33] A. Nowicki - (6), p.7.[98, 2012; Theorem 1.1] R. Mestrovic - (30), p. 12.[97, 2012; Theorem] R. Mestrovic (also see [30, Corollary1]) - (42), p. 15.[97, 2012; Proposition] R. Mestrovic - (47) and (48), p. 17.[115, 2014; Section 5, Theorem 5.3] E. Rowland and R. Yassawi- (44), p. 16.

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