Finite geometry codes,generalized Hadamard matrices,

and Hamada and Assmusconjectures

Vladimir D. Toncheva

Department of Mathematical Sciences

Michigan Technological University

Houghton, Michigan 49931, USA

tonchev@mtu.edu, www.math.mtu.edu/tonchev

aSpeakers participation sponsored by Fulbright Grant #1868.

Finite geometry codes, generalized Hadamard matrices, and Hamada and Assmus conjectures p. 1/23

Overview

All generalized Hadamard matrices of order16 over a group of order 4 are classified up toequivalence.

The quaternary codes spanned by thesematrices and the binary linear codes spannedby the incidence matrices of relatedsymmetric nets are computed and classified.

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The binary codes include the affine geometry[64, 16, 16] code spanned by the planes inAG(3, 4) and two new codes that supportnon-isomorphic designs with the same 2-rankas the classical affine design in AG(3, 4),hence provide counter-examples to Hamadasand Assmus conjectures.

Many of the F4-codes spanned bygeneralized Hadamard matrices yieldquantum error-correcting codes, includingsome codes with optimal parameters.

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Designs

A t-(v, k, ) design D is a set X of v pointstogether with a collection B of b k-subsets of Xcalled blocks such that every t-subset of X iscontained in exactly blocks.

A design is symmetric if v = b.

Two designs are isomorphic if there exists abijection between their point sets that maps theblocks of the first design into blocks of thesecond design.

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Incidence Matrices

An incidence matrix of D is a b v (0, 1) matrixA = (aij) with rows indexed by the blocks, andcolumns indexed by the points, where aij = 1 ifthe ith block contains the jth point and aij = 0otherwise.

The dual design D of D is the design withincidence matrix AT .

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Resolvable Designs

A parallel class in a t-(qk, k, ) design is a set ofq pairwise disjoint blocks.A resolution is a partition of the collection ofblocks into disjoint parallel classes.A design is resolvable if it admits a resolution.A resolvable design is affine resolvable or affine,if every two blocks that belong to different parallelclasses of R intersect in a constant number of = k2/v points.The classical affine 2-(qn, qn1, (qn1 1)/(q 1))design has the hyperplanes in the affinegeometry AG(n, q) as blocks.

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Symmetric Nets

A symmetric (, q)-net is a symmetric1-(q2, q, q) design D such that both D and Dare affine.

A symmetric (, q)-net is class-regular if it admitsa group of automorphisms G of order q that actstransitively on every point and block parallelclass.

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The Classical Nets

The classical class-regular (q, q)-net, where q is aprime power, is obtained from the 3-dimensionalaffine space AG(3, q) over the field of order q asfollows:

Choose a class P of q2 parallel lines in AG(3, q),that is, P consists of a given 1-dimensionalvector subspace and its cosets in GF (q)3, andconsider as blocks of the net the q3 planes inAG(3, q) that do not contain any line from P. Thegroup of bitranslations G in this case is anelementary Abelian group of order q.

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Generalized Hadamard matrices

A generalized Hadamard matrix H(,G) = (hij)over a group G of order q is a q q matrix withentries from G with the property that for every i,j, 1 i < j q, the multi-set

{hish1js | 1 s q}

contains every element of G exactly times.

A generalized Hadamard matrix over themultiplicative group of order two G = {1,1} isan ordinary Hadamard matrix.

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GH Matrices and Nets

Every generalized Hadamard matrixH = H(,G) over a group G of order qdetermines a class-regular symmetric (, q)-netN as follows: let G be a group of q by qpermutation matrices isomorphic to G, and let be an isomorphism between G and G. Replacingeach element hij of H by (hij) gives a(0, 1)-incidence matrix of a class-regularsymmetric (, q)-net N .

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EXAMPLE

H(2, 2) =

0

B

B

B

B

B

@

1 1 1 1

1 1 1 1

1 1 1 1

1 1 1 1

1

C

C

C

C

C

A

, 1

0

@

1 0

0 1

1

A , 1

0

@

0 1

1 0

1

A ,

H(2, 2) N =

0

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

@

1 0 1 0 1 0 1 0

0 1 0 1 0 1 0 1

1 0 1 0 0 1 0 1

0 1 0 1 1 0 1 0

1 0 0 1 1 0 0 1

0 1 1 0 0 1 1 0

1 0 0 1 0 1 1 0

0 1 1 0 1 0 0 1

1

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

A

.

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Class-Regular (q, q)-Nets: Classicifation

q Group Class-regular nets Total # nets2 Z2 1 13 Z3 2 44 Z4 13 239

4 Z2 Z2 226 239

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The Class-Regular (4, 4)-Nets

There are 13 non-isomorphic (4, 4)-nets withgroup Z4.

There are 226 non-isomorphic (4, 4)-nets withgroup Z2 Z2.

These nets give rise to 13 inequivalentgeneralized Hadamard matrices of order 16 overthe cyclic group Z4 of order 4, and 226 suchmatrices over the elementary Abelian groupZ2 Z2.

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Hamadas Conjecture

Conjecture (N. Hamada, 1973):

A geometric design having as points and blocksthe points and subspaces of a given dimensionof a finite affine or projective space over GF (q) ischaracterized as the unique design with thegiven parameters having minimum q-rank of itsincidence matrix.

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The Proven Cases

Hamadas conjecture was proved to be true inthe following cases:

Classical (hyperplane) designs in AG(n, 2)and PG(n, 2) (Hamada and Ohmori 75);

Lines in PG(n, 2) and AG(n, 3) (Doyen,Hubaut and Vandensavel 78);

Planes in AG(n, 2) (Teirlinck 80).

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Counter-Examples

The only previously known counter-examples ofHamadas conjecture were five 3-(32, 8, 7)designs supported by extremal doubly-evenself-dual [32, 16, 8] codes (one being the secondorder Reed-Muller, or affine geometry code), andtheir derived 2-(31, 7, 7) designs supported by theshortened codes, all having 2-rank 16 (V.D.Tonchev 1986).

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Assmus Conjecture

Assmus Conjecture:

Hamadas conjecture is true for designs withclassical parameters.

Theorem. (V.D. Tonchev 1999).The Assmus conjecture is true for generalizedincidence matrices with entries over GF (q).

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Binary Codes from (4, 4)-Nets

The binary linear codes spanned by the 64 64incidence matrices of the (4, 4)-nets werecomputed and classified.

Three codes (C1, C20 and C36), have the followingweight enumerator:

W (y) = 1 + 84y16 + 3360y24 + + y64.

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New Counter-Examples

The vectors of weight 16 in each of the codesC1, C20 and C36 support an affine 2-(64, 16, 5)design.

The design in C1 is isomorphic to the classicaldesign of the planes in AG(3, 4).

The 2-(64,16,5) designs in C20 and C36 arethe only known designs with classicalparameters that are not geometric but havethe same p-rank as a geometric design.

These designs are counter-examples to bothHamadas and Assmus conjectures.

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Quantum Codes

The hermitian product ofx = (x1, . . . , xn), y = (y1, . . . , yn) over GF (4):

(x, y) = x1y2

1 + x2y2

2 + + xny2

n

Theorem. Calderbank, Rains, Shor and Sloane, 1998:

A hermitian self-orthogonal GF (4)-code C oflength n with dual distance d(C) (where C isthe hermitian dual code of C) yields a quantumerror-correcting code with parameters[[n, k = n 2dimC, d = d(C)]].

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Quantum Codes from GH Matrices

Normalizing a generalized Hadamard matrixof order 16 over Z2 Z2 gives a generatormatrix of a self-orthogonal code of length 15over GF (4).

Among these codes, 150 are hermitianself-orthogonal, hence give rise to quantumcodes.

The matrix related to the classical net yieldsan optimal quantum [[15, 11, 2]] code.

Several matrices give optimal quantum[[15, 7, 3]] codes.

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REFERENCES

A.R. Calderbank, E.M. Rains, P.W. Shor, and N.J.A. Sloane, Quantum errorcorrection via codes over GF (4), IEEE Trans. Inform. Theory 44 (1998),13691387.

N. Hamada, On the p-rank of the incidence matrix of a balanced or partiallybalanced incomplete block design and its applications to error-correcting codes,Hiroshima Math. J. 3 (1973), 153226.

M. Harada, C. Lam and V.D. Tonchev, Symmetric (4, 4)-nets and generalizedHadamard matrices over groups of order 4, Designs, Codes and Cryptography 34(2005), 71-87.

V.D. Tonchev, Quasi-symmetric 2-(31,7,7) designs and a revision of Hamadasconjecture, J. Combin. Theory, Ser. A 42 (1986), 104-110.

V.D. Tonchev, Linear perfect codes and a characterization of the classical designs,Designs, Codes and Cryptography 17 (1999), 121-128.

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Thank You!

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OverviewDesignsIncidence MatricesResolvable DesignsSymmetric NetsThe Classical Nets Generalized Hadamard matricesGH Matrices and Nets EXAMPLE Class-Regular $(q,q)$-Nets:ClassicifationThe Class-Regular $(4,4)$-NetsHamada's ConjectureThe Proven Cases Counter-ExamplesAssmus ConjectureBinary Codes from $(4,4)$-NetsNew Counter-ExamplesQuantum CodesQuantum Codes from GH Matrices REFERENCESThank You!