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Entropy of Deterministic Networks and Network Ensembles Justin P. Coon in collaboration with A. Cika, C. P. Dettmann, O. Georgiou, D. Simmons, P. J. Smith Short Course on Complex Networks and Point Processes with Applications

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Page 1: Entropy of Deterministic Networks and Network Ensembles · Theorem (Compression via Source Coding) ... Bernoulli Experiment ... Entropy of Deterministic Networks and Network Ensembles

Entropy of Deterministic Networksand Network Ensembles

Justin P. Coon

in collaboration withA. Cika, C. P. Dettmann, O. Georgiou, D. Simmons, P. J. Smith

Short Course on Complex Networksand Point Processes with Applications

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Overview

I History and motivation

I Complexity measures for deterministic networks

I Introduction to entropy

I Entropy measures for deterministic networks

I Entropy of nonspatial network ensembles

I Entropy of spatial network ensembles

I Applications

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History and Motivation

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A Brief History

The task of measuring graph structure has been a worthwhileobjective for a number of years in many disciplines, includingchemistry, sociology, computer science, and ecology. Effortsto characterise complexity in networks have gathered pace sincethe dawn of the Internet.

050

100150200250300350400450

20002001200220032004200520062007200820092010201120122013201420152016

NumberofPapersCi0ng"GraphEntropy"or"NetworkEntropy"

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A Brief History

Mathematical studies of graph structure and complexity dateback to Rashevsky (1955) and Mowshowitz (1968).

This formalism was taken up in earnest within the biological andchemical sciences to classify chemical structures by Bonchev etal (1977, 1983).

At the turn of the millennium, the network physics communitybegan to explore network entropy. Notable work was published byPark and Newman (2004), Anand and Bianconi (2009).

Around the same time, graph entropy was employed to studysocial networks (Butts, 2001) and ecological networks (Sole etal, 2001; Ulanowicz, 2004).

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Why study network entropy?

1 km

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Why study network entropy?

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Why study network entropy?

Local behaviour 7→ Global complexity

1 km

1 km

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Complexity Measures for Deterministic Networks

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Measuring Complexity through Structure

Substructure Count

Idea: Complex networks contain a rich substructure.

Example: Complexity is quantified as the subgraph count

complexity(G ) :=

|E|∑

k=0

subgraphCountk(G )

Notation and terminology: G = (V, E) and we use “graph” and“network” interchangeably.

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Measuring Complexity through Generation

Generative

Idea: A large number of operations must be performed on a set ofprotographs to construct a complex network.

Example: Given a set of protographs isomorphic to stars,complexity is quantified as the number of unions and intersectionsof these elemental graph structures required to generate the edgeset of a given network.

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Measuring Complexity through Encoding

Encoding

Idea: A large number of yes/no questions (bits) are required todescribe a complex network.

Example: A four-node network would require at most six bits todescribe the topology. A 400 node network would require at most79,800 bits to describe its topology. (Different encoding schemeslead to different results.)

Do better encoding schemes exist?

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A Journey into Theoretical Computer Science

Encoding measures are related to Kolmolgorov complexity andminimum description length.

Definition (Kolmolgorov Complexity)

The Kolmolgorov complexity of an object is defined as the smallestpossible description of that object using a fixed, universaldescription language.

Definition (Minimum Description Length)

The principle that the best encoding of a dataset is the one thatcompresses it the most.

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Encoding Example

1

2

3

4

5

6

7

8

This graph has 28 possible edges, and hence we can encode the graphusing a 28-bit string. Alternatively, we could list the edges as vertex pairs(in binary) and terminate on one end with the sequence 111111 as follows

111111 001100 001101 010111

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Touching on Communication/Information Theory

Theorem (Compression via Source Coding)

Consider a discrete memoryless source with symbols denoted bythe random variable X . Suppose groups of J symbols are encodedinto sequences of N bits. Let Pe be the probability that a block ofsymbols is decoded in error. Then Pe can be made arbitrarily smallif

R =N

J≥ entropy(X ) + ε

for some ε > 0 and J sufficiently large. Conversely, if

R ≤ entropy(X )− ε

then Pe becomes arbitrarily close to 1 as J grows large.

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Making the Link: From Complexity to Entropy

Complexity 7→ MDL 7→ Entropy

Entropy has a rich mathematical history in physics and informationtheory. Hence, this formalism provides us with a much morecomplete set of tools for analysing network structure andcomplexity.

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Introduction to Entropy

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Shannon Entropy

Shannon Entropy

Shannon entropy is defined with respect to a probability distributionp0, p1, . . . as

H(p0, p1, . . .) = −c∑

i

pi log pi

where c is a constant that determines the base of the log.

When p0, p1, . . . describes the distribution of a discrete randomvariable X , we often write

H(X ) = E[− logP(X )]

If X is a continuous random variable with density p(x), the differentialentropy is defined as

H(X ) = E[− log p(X )] = −∫

supp(X)

p(x) log p(x)dx

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Properties of Shannon EntropyI Positive and Finite

0 ≤ H(X ) ≤ log |supp(X)|

I Concave

H(λp0 + (1− λ)p1) ≥ λH(p0) + (1− λ)H(p1), λ ∈ [0, 1]

I Joint Entropy

H(X ,Y ) = E[− logP(X ,Y )]

I Conditional Entropy

H(X |Y ) = E[− logP(X |Y )]

I Independence

H(X |Y ) = H(X )⇒ H(X ,Y ) = H(X ) + H(Y |X ) = H(X ) + H(Y )Entropy of Deterministic Networks and Network Ensembles 11 September, 2017

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Example: Bernoulli Experiment

Consider a coin toss where the probability of a heads is p and theprobability of a tails is 1− p. The entropy is

H(p) = −p log p − (1− p) log(1− p)

0.2 0.4 0.6 0.8 1.0 p

0.2

0.4

0.6

0.8

1.0

H(p)

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Entropy Variants

Other definitions of entropy exist, which are suitable for use in thecontext of ensembles, but comparatively little work related to theentropy of graph ensembles using these measures has beenreported in the literature.

Renyi Entropy

Renyi entropy of order α is defined for the probability distributionp0, p1, . . . as

Hα(p0, p1, . . .) :=1

1− α log

(∑

i

pαi

).

It generalises Shannon entropy (H = limα→1 Hα), max-entropy(H0), collision entropy (H2), and min-entropy (limα→∞Hα). It isSchur concave.

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Entropy Variants

Von Neumann Entropy

The von Neumann entropy of a quantum mechanical systemdescribed by the density ρ is defined as

H(ρ) := −tr(ρ ln ρ)

For the eigendecomposition ρ =∑

i ωi |i〉 〈i |, the von Neumannentropy takes the Shannon form

H(ρ) = −∑

i

ωi lnωi

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Entropy Measures for Deterministic Networks

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General Approach

The key to using entropy as a measure of structural information orcomplexity in the context of networks is to define a probabilitydistribution on an appropriate set of graph invariantsassociated with the network.

Take a graph invariant Z (i.e., a property of the graph that isinvariant under isomorphisms) and define an equivalencerelation that induces a set of equivalence classes Zi. Ingeneral, we can use the Shannon entropy formalism to define theentropy of the graph with respect to that equivalence relation as

H(G ) := −∑

i

|Zi ||Z| log

|Zi ||Z|

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Topological Information Content

Consider the automorphism group of a graph, i.e., the set ofgraphs derived from vertex permutations whereby edges in theoriginal graph are contained in the set of edges in the permutedgraph. The equivalence classes of the graph under automorphismsare called vertex orbits. Let Vi denote the ith orbit, and K is thenumber of different orbits. Rashevsky (followed by Mowshowitz)formalised the notation of topological information content as

Ha(G ) := −K∑

i=1

|Vi ||V| log

|Vi ||V|

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Automorphism Group Example

12

3

4

5

6

78

Automorphisms

(1)(2)(3)(4)(5)(6)(7)(8)(1 8)(2 7)(3)(4)(5)(6)(1)(2)(3 5)(4 6)(7)(8)(1 8)(2 7)(3 5)(4 6)

Vertex orbits

(1 8)(2 7)(3 5)(4 6)

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Chromatic Information Content

The chromatic number of a graph, denoted by χ(G ), is thesmallest number of colours required to colour the vertices suchthat no two adjacent vertices share the same colour. LetV = Vi |1 ≤ i ≤ χ(G ) denote the a chromatic decomposition ofa graph. Then V forms a set of equivalence classes, and we candefine the chromatic information content of a graph as(Mowshowitz, 1968)

Hc(G ) := minV

χ(G)∑

i=1

|Vi ||V| log

|Vi ||V|

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Radial Centric Information Content

The eccentricity σv of a vertex v is the maximum distance (pathlength) from that vertex to any other vertex in the network. LetVσ denote the set of vertices with eccentricity σ. Denote thediameter of the graph as D. Then Vσ form a set of equivalenceclasses, and we can define the radial centric information contentof a graph as (Bonchev, 1983)

Hr (G ) := −D∑

σ=1

|Vσ||V| log

|Vσ||V|

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Vertex Degree Information Content

Let Vδ denote the set of vertices with degree δ. Denote themaximum degree of the vertices by δ. Then Vδ form a set ofequivalence classes, and we can define the vertex degreeinformation content of a graph as (Bonchev, 1983)

Hv (G ) := −δ∑

δ=0

|Vδ||V| log

|Vδ||V|

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Parametric Graph Entropy

The measures discussed previously are intrinsic to the network inquestion. Extrinsic measures can also be defined by placing avalue of some description to features of the graph. The mostgeneral and accessible approach that has been proposed is to usean information function f : V 7→ R+ that acts on the individualvertices. Maintaining the need for a probabilistic interpretationyields (Dehmer, 2011)

Hp(G ) := −|V|∑

i=1

f (vi )∑|V|j=1 f (vj)

logf (vi )∑|V|j=1 f (vj)

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Different Interpretations for Different Measures

Example (Vertex Degree Information Content vs. ParametricDegree Entropy)

Let f (vi ) := δ(vi ). For the cycle graph CN with N vertices, theparametric degree entropy is logN. Yet, there is a single nonemptydegree equivalence class corresponding to δ = 2, and thus the vertexdegree information content is zero.

The vertex degree information content is the entropy of the degreedistribution. It is upper bounded by log(1 + δ).

The parametric degree entropy, which can be written as

Hp,δ(G ) = −|V|∑

i=1

δ(vi )

2|E| logδ(vi )

2|E|

quantifies the uniformity of the vertex degrees across the set V.

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Entropy of Nonspatial Network Ensembles

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Refining the Definition for Ensembles

When working with ensembles of networks, a probabilitydistribution P can often be naturally defined on the set ofgraphs. The graph G then becomes a random variable, andthe entropy of the ensemble G is defined as

H(G) := E[− logP(G )]

in its most general form.

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Erdos-Renyi (ER) Ensembles

Introduced in 1959, this model concerns an ensemble ofgraphs GN,E , where each graph is formed of N vertices and Eedges. The random graph (i.e., random variable) GN,E isdrawn uniformly from this set.

There are(N(N−1)/2

E

)graphs in the ensemble. Hence, the

entropy is

H(GN,E ) = log

(N(N − 1)/2

E

)

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Gilbert Ensembles

Also introduced in 1959, this model concerns an ensemble of graphs GN,p,where each graph is formed of N vertices and where each edge exists withprobability p independent of all other edges. This is often referred to asthe ER ensemble since Erdos and Renyi considered the model as well.

The entropy of the random graph GN,p is equivalent to the joint entropyof the edges, i.e.,

H(GN,p) = H(X1,2,X1,3, . . . ,XN−1,N)

where P(Xi,j = 1) = p. Independence implies

H(GN,p) =∑

i<j

H(Xi,j) =N(N − 1)

2H(p)

where H(p) = −p log p − (1− p) log(1− p).

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Exponential Random Graphs (ERG) Ensembles

The ERG model, originally studied in the context of social networkanalysis (Holland and Leinhardt, 1981; Frank and Strauss, 1986),assumes a random graph G has a distribution

P(G ) =exp(

∑i θizi (G ))

κ(θi)

where θi are model parameters and zi (G ) are statistical observables.The parameter κ(θi) is a normalisation constant that ensures∑

G∈G P(G ) = 1.

From an equilibrium statistical physics perspective, this distribution canbe derived more constructively...

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Exponential Random Graphs (ERG) Ensembles

To derive the ERG distribution using the principles of equilibriumstatistical mechanics, we seek the function P that maximises the Gibbsentropy functional

S [P] = −∑

G∈GP(G ) lnP(G )

subject to the constraints

〈zi 〉 =∑

G∈GP(G )zi (G ) and 1 =

G∈GP(G ).

Forming the Lagrangian with multipliers λi, yields the requiredcondition

∂P(G )

S [P] + λ0

(1−

X∈GP(X )

)+∑

i

λi

(〈zi 〉 −

X∈GP(X )zi (X )

)= 0

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Exponential Random Graphs (ERG) Ensembles

Functional differentiation results in

lnP(G ) + 1 + λ0 +∑

i

λizi (G ) = 0

which leads to

P(G ) =exp (−∑i λizi (G ))

Z (λi)with Z (λi) = e1+λ0 .

To recover the relationship to the ERG model and equilibrium statisticalmechanics, note that λi = −θi , Z = κ is the partition function, and∑

i λizi (G ) is the Hamiltonian. The entropy of the ERG ensemble is

H(G) =

(1 + λ0 +

i

λi 〈zi 〉)

log e

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Entropy of Spatial Network Ensembles

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Basic Definitions

Definition (Point Process)

A point process is a mathematical model for a set of randomdistributed points in some space.

Definition (Binomial Point Process (BPP))

Let λ denote a spatial density defined on a set B ⊆ S. A pointprocess with N points independently and identically distributed inB according to λ is called a binomial point process.

Definition (Pair Distance Distribution)

The pair distance distribution gives a statistical description ofthe distance between two arbitrary points in some space.

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Spatial Network Model

I Embed N nodes randomly (BPP) in Kd ⊂ Rd .

I Connect nodes i and j , which are separated by a distance ri ,j ,with probability p(ri ,j).

I Doing this many times yields the a graph ensemble G.

Example (Erdos-Renyi) Example (Spatial)

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Upper Bound on Entropy

Theorem

Given a point process situated in Kd ⊂ Rd admitting the pair distancedensity f (r), and assuming a pair connection function p(r), the entropyof the resulting graph ensemble G satisfies

H(G) ≤ N(N − 1)

2H(p)

where

p =

∫ D

0

p(r)f (r)dr

and D is the diameter of Kd .

Proof.The proof uses the correspondence between a graph and its edge set. Thebound follows from an argument based on the concavity of entropy.

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Lower Bound on Entropy

Theorem

Given a point process situated in Kd ⊂ Rd admitting the pair distancedensity f (r), and assuming a pair connection function p(r), the entropyof the resulting graph ensemble G satisfies

H(G) ≥ N(N − 1)

2

∫ D

0

H(p(r))f (r)dr =: H(G|P)

The lower bound H(G|P) is called the conditional entropy of theensemble G given the distribution of the point process P.

Proof.

The proof follows from the classical definition of conditional entropy (inthe Shannon sense) and the fact that conditioning cannot increaseuncertainty.

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Consequences of the Bounds

The upper and lower bounds yield the following immediate results:

Property 1 The entropy of an ensemble arising from a softconnection function scales like N2.

Property 2 Classical random geometric graphs (with a hardconnection function) have zero conditional entropy.

Property 3 The reduction in uncertainty of the ensemble(topology) given the statistical properties of thepoint process quantifies the mutual informationbetween G and P

I (P;G) := H(G)− H(G|P)

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Controlling Complexity as Networks Scale

If a system has a typical connection range r0, it ispossible to derive scaling laws that provide insightabout how to control the complexity of a networkensemble as the number of vertices grows large.

Test function:

p(r) = exp (−(r/r0)η)

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Controlling Complexity as Networks Scale

Substituting into (2), we have

H(G) ' (n2)X

k=0

n2

k

pk(1 p)(

n2)k log

pk(1 p)(

n2)k

=

n

2

H(p) (9)

whereH(p) = p log p (1 p) log(1 p) (10)

is the binary entropy function.The structure of (9) is somewhat predictable. Indeed, if

pi,j = p for all i < j, G is an ER graph. The entropy ofER graphs is well known to be

HER(G) =

n

2

H(p). (11)

So by assuming pair distances are independent in the RGG case,we obtain an ER-like result, but where the pairwise connectionprobability is averaged over the pair distance distribution.

The independence assumption that led to (6) deserves com-ment. In fact, it can easily be reasoned that pair distances arenot necessarily independent when the network domain is finite.Consider a simple example of three nodes positioned on theinterior of a circle. Suppose node i resides on the boundary,and nodes j and k are positioned such that ri,j and ri,k are bothapproximately D, the diameter of the circle. Clearly, nodes jand k must lie very close to each other, and so knowledge ofri,j and ri,k provides statistical information about rj,k.

If we return to the original definition of the entropy of Gprovided in (2), we can make use of the fact that, for a givennumber of nodes n, the distribution of G is equivalent to thedistribution of the edge set. Let Xi,j denote a Bernoulli randomvariable that models the existence (or not) of edge (i, j). Itfollows that we can write

H(G) = H(X1,2, X1,3, . . . , Xn1,n) X

i<j

H(Xi,j) (12)

where equality holds if all Xi,j are independent. The randomvariable Xi,j is physically related to nodes i and j, but sincepair distance information is not included in (12), we can write

P(Xi,j) =

Z D

0

P(Xi,j |ri,j)f(ri,j) dri,j = p (13)

for all i < j. It follows that

H(G)

n

2

H(p). (14)

Thus, the approximation given in (9) is an upper bound onH(G). It is important to note that although we have arrived atthis bound by simple means via (12), this approach abstractsthe pair distance considerations taken earlier, and as a result itis perhaps less physically intuitive. Indeed, without consideringthe effects of the embedding geometry, it is not immediatelyobvious that (12) does not hold with equality.

Fig. 1. Entropy of an RGG with n = 5 nodes and pair connection functionswith typical range r0 for a unit square. Solid lines: upper bound h(n). Markers:numerical simulations for H(G).

IV. EFFECT OF FADING CHANNELS ON UNCERTAINTY

We now study topological uncertainty in fading channelsby analyzing the entropy approximation (bound) given inthe previous section, which we denote by h(n) =

n2

H(p)

to aid exposition. Our focus is on systems that experiencediffuse multipath propagation. In other words, we investigatethe case when the small-scale fading model is Rayleigh, andthe pairwise connection function that will form the basis of thisanalysis is that given in (1).

Before proceeding with a detailed analysis, it is instructive toexamine Fig. 1, which shows the graph entropy H(G) and thebound h(n) for a five-node network plotted against the typicalconnection range r0 for path loss exponents = 2, 3, 4,1.The bounding geometry is a unit square in this example. A fewimportant things can be noted from the figure. First, the boundh(n) is relatively accurate, particularly for very soft connectionfunctions (e.g., = 2) and for small/large r0. Second, theentropy decreases as r0 ! 0 and r0 ! 1. Third, the maximaof h(n) and H(G) appear to coincide fairly well2. Motivatedby these observations, we will analyze the small/large typicalconnection range regimes and develop scaling laws that linkr0 and the number of nodes in the network n to the entropybound h(n).

A. Small Typical Connection Range

Our main results relate to the case where the typical con-nection range decreases as the number of nodes in the networkgrows large (see Fig. 2). Intuitively, one can recognize thatit is possible for the entropy bound to tend to zero in sucha case, which would signify that the network is completelydisconnected3. Hence, it is of fundamental interest to ascertain

2This is an interesting property that warrants further study, but we refrainfrom addressing it in this paper.

3Of course, h(n) = 0 may also signify that the network is completelyconnected (a complete graph); but this is impossible in the scaling regimeconsidered here (r0 ! 0). We will consider r0 ! 1 in the next section.

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Controlling Complexity as Networks Scale

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Statistical Characterisation of Sets

Definition (Set Covariance)

In d dimensions, the set covariance of a convex set Kd is given by

cKd(r) =

Rd

1Kd(x)1Kd

(x− r)dx, r ∈ Rd

where 1Kd(x) is the indicator function for x ∈ Kd .

Definition (Isotropised Set Covariance)

The isotropised set covariance is given by

cKd(r) =

Sd−1

cKd(ru)U(du), r ≥ 0

where u is a vector denoting a point on the unit sphere Sd−1.

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Another Look at Pair Distance

Theorem (Pair Distance Density Function)

The pair distance probability density function is given by

f (r) =2πd/2rd−1cKd

(r)

Γ(d/2) vol(Kd)2.

Theorem (Small Argument)

For small distances, the pair distance density can be approximatedto first order as

f (r) ' vol(Kd)− r limr→0

Sd−1

vol((Kd ∩ (Kd + ru))u⊥)U( du)

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Scaling Law in 2D

Theorem

As N →∞, the entropy of a graph in K2 can be bounded awayfrom zero only if

r20 log

(1

r0

)= Ω

(1

N2

).

The entropy bound will tend to a limit `h > 0 as N →∞ if

r0(N) = exp

(1

2Wm

(− 2`hu2N(N − 1)

))

=

(`h

u2N2logN

) 12(

1 + O

(1

logN

))

where Wm(x) is the lower branch (−1/e ≤ x < 0 and Wm ≤ −1)of the solution to x = W expW .

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Scaling Law in 3D

Theorem

As N →∞, the entropy of a graph in K3 can be bounded awayfrom zero only if

r30 log

(1

r0

)= Ω

(1

N2

).

The entropy bound will tend to a limit `h > 0 as N →∞ if

r0(N) = exp

(1

3Wm

(− 2`hu3N(N − 1)

))

=

(`h

u3N2logN

) 13(

1 + O

(1

logN

))

where u3 = 4πΓ(3/η)/(ηV (K3)) is the fractional volume of a softunit ball.

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Maximum Entropy Pair Connection Function(Conditional Entropy)

As with nonspatial networks, it is natural to study the maximum entropyproperties of the pair connection function for spatial ensembles. Onemust first define a set of meaningful constraints

∫ D

0

θ`(r)p(r)dr , ` = 1, . . . , L

Solving the associated constrained variational problems yields the entropymaximising function

p(r) =1

eψ(r) + 1

where

ψ(r) =2

N(N − 1)f (r)

L∑

`=1

λ`θ`(r)

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Applications

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Applications: Measuring Complexity in Practice

Example (Wireless Networks)

The pair connection function in wireless networks is typically well defined.Hence, we can quantify how complex engineered networks are.

=

==

0.0 0.5 1.0 1.5 2.0r0.0

0.2

0.4

0.6

0.8

1.0

p(r)

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Applications: Measuring Complexity in Practice

Example (Airline Networks)

Flight path data is available for primary airports. Here, we show that the(conditional) entropy of the primary conterminous US airline network isnearly maximal, despite the difference in pair connection functions.

0 500 1000 1500 2000r0.00

0.02

0.04

0.06

0.08

0.10

0.12p(r)

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Applications: Communication Network Protocols

Example (Routing Protocol Design)

The parametric graph entropy model has been adopted to design routingprotocols in communication networks. The idea is to design atime-dependent stability metric for each link (S t

l ). A probabilitydistribution for each link in the routing table of the ith node can then bedefined

pl =S tl∑

l∈RtiS tl

and the entropy at time t and the ith node calculated to be

Hi (t) = − 1

log |Rti |∑

l

pl log pl .

Routing will be executed such that stability (measured through nodeentropy) is maximised.

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Applications: Graph Compression

Example (Compressing Graphical Structures)

Choi and Szpankowski (2012) studied the problem of structural entropy(defined with respect to isomorphic graphs) in Gilbert graph ensemblesand used this to derive structural compression algorithms. Applicationsinclude the compression of biological or medical datasets andtopographical maps. It was shown that for p lnN/N and1− p lnN/N, the structural entropy of the ensemble is

HS(GN,p) ∼ N(N − 1)

2H(p)− logN!.

The compression algorithm is iterative: the number of neighbours of avertex are stored and the remaining vertices are partitioned into disjointsets depending on their relationship to this vertex; a neighbour of theoriginal vertex is then selected and the process repeats, wherepartitioning is executed by taking into account all parent vertices.

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