Peter´ Csikvari´ Counting in Sparse Graphs Lecture note Contents 1 Statistical Physical Models 1 1.1 Combinatorics and statistical physics . .1 1.2 Factor graphs . .5 1.3 Tutte polynomial . .8 2 Extremal Regular Graphs 12 2.1 Extremal regular graphs . 12 2.2 The case of Kd;d ................................ 13 2.3 The case of Kd+1 ............................... 16 2.4 The case of Td ................................. 18 3 Sidorenko's conjecture 20 3.1 Sidorenko's conjecture . 20 3.2 Graphs with Sidorenko's property . 21 3.3 Sidorenko's conjecture with fixed target graph . 23 4 Graph Polynomials 27 4.1 Adjacency matrix and eigenvalues . 27 4.2 Matching polynomial . 35 4.3 Chromatic polynomial . 43 5 Empirical Measures 46 5.1 Benjamini{Schram convergence and convergence of measures . 47 5.2 Moments and homomorphism numbers . 49 5.3 Kesten-McKay measure . 54 5.4 McKay's theorem . 55 6 Correlation Inequalities 60 6.1 Gibbs-measures . 60 6.2 Positive correlation . 60 i 7 Graph Covers 70 7.1 Two ideas, one method . 70 7.2 Bipartite double cover . 71 7.3 Independent sets, matchings and 2-covers . 73 7.4 Ruozzi's theorem . 75 7.5 Large girth phenomena . 76 7.6 Matchings . 76 8 Bethe Approximation 79 8.1 Covers of factor graphs . 83 9 Stable Polynomials 86 9.1 Multivariate polynomials . 86 10 Entropy 98 10.1 Information and counting . 98 10.2 Basic properties of entropy . 98 10.3 Matchings: Br´egman'stheorem . 102 10.4 Homomorphisms . 104 Bibliography 107 ii 1. Statistical Physical Models 1.1 Combinatorics and statistical physics The counting problems studied in this book are directly related to statistical physical models. These models in turn provide a large amount of motivation for the investigated phenomenons. Here we survey these models and their relationships with each other. 1.1.1 Monomer-dimer and dimer model For a graph G let M(G) be the set of all matchings. Recall that a matching M of G is simply a set of edges such that now two edges in the set intersect each other. When this set has k edges, then we say that it is a k-matching or alternatively, the matching M is of size k. For a λ > 0 we can associate a probability space on M(G) by choosing a random matching M as follows: λjMj (M = M) = ; P M(G; λ) where M(G; λ) is the normalizing constant: X M(G; λ) = λjMj: M2M(G) This model is the monomer-dimer model. The name has the following origin. In statistical physics the vertices of the graph represent particles, and edges represent some sort of interaction between certain pair of particles. A dimer is then simply a pair of particles where the interaction is active. Supposing that one particle can be in active with at most one other particles we get that dimers form a matchings. The uncovered vertices are then called monomers. We say that M(G; λ) is the partition function of the monomer-dimer model. In mathematics it is called the matching generating function. Let mk(G) denote the number of k-matchings. Then X k M(G; λ) = mk(G)λ : k Note that the sum runs from k = 0 as the empty set is a matching by definition. Naturally, once we introduced a probability distribution we can ask various natural question like 1 what EjMj is. It is not hard to see that X jMjλjMj λM 0(G; λ) jMj = = : E M(G; λ) M(G; λ) M2M(G) If G has 2n vertices then we call an n-matching a perfect matching as it covers all vertices. The dimer model is the model where we consider a uniform distribution on the perfect matchings. Clearly, a dimer model is a monomer-dimer model without monomers. The number of perfect matchings is pm(G). With our previous notation pm(G) = mn(G). 1.1.2 Hard-core model For a graph G let I(G) be the set of all independent sets. Recall that an independent set is a subset I of the vertices such that no two element of I is adjacent in G. Here the vertices of G represent possible places for particles that repulse each other so no two adjacent vertex can be occupied by particles. For a λ > 0 we can associate a probability space on I(G) by choosing a random matching I as follows: λjIj (I = I) = ; P I(G; λ) where I(G; λ) is the normalizing constant: X I(G; λ) = λjMj: I2I(G) Then I(G; λ) is the partition function of the hard-core model. In mathematics it is called the independence polynomial of the graph G. Let ik(G) denote the number of independent sets of size k. Then n X k I(G; λ) = ik(G)λ : k=0 Note that the sum runs from k = 0 as the empty set is an independent set by definition. Similar to the case of the matchings we have X jIjλjIj λI0(G; λ) jIj = = : E I(G; λ) I(G; λ) I2I(G) 1.1.3 Ising-model In the case of the Ising-model the vertices of the graph G represent particles. These particles have a spin which can be up (+1) or down (−1). Two adjacent particles have an interaction eβ if they have the same spin, and e−β if they have different spin. Suppose also that there is an external magnetic field that breaks the symmetry between +1 and 2 −1. This defines a probability distribution on the possible configurations as follows: for a random spin configuration S: 0 1 1 X X (S = σ) = exp βσ(u)σ(v) + B σ(u) ; P Z @ A (u;v)2E(G) u2V (G) where Z is the normalizing constant: 0 1 X X X ZIs(G; B; β) = exp @ βσ(u)σ(v) + B σ(u)A : σ:V (G)→{−1;1g (u;v)2E(G) u2V (G) Similarly to the previous cases, Z is the partition function of the Ising-model. When β > 0 we say that it is a ferromagnetic Ising-model, and when β < 0 then we say that it is an antiferromagnetic model. It turns out that the model behaves very differently in the two regimes even if we only consider extremal graph theoretic questions. 1.1.4 Potts-model and random-cluster model Potts-model is a generalization of the Ising-model. Again the vertices of the graph G represent particles, but this time their spin or state can be one of q different states, i. e., σ : V (G) ! [q]. Two adjacent particles have an interaction eβ if they are in the same state, and no interaction otherwise. This defines a probability distribution on the possible configurations as follows: for a random configuration S: let 1(statement) = 1 if the statement is true and 0 otherwise, then 0 1 1 X (S = σ) = exp β1(σ(u) = σ(v)) ; P Z @ A (u;v)2E(G) where Z is the normalizing constant: 0 1 X X ZPo(G; q; β) = exp @ βI(σ(u) = σ(v))A : σ:V (G)![q] (u;v)2E(G) Similarly to the previous cases, Z is the partition function of the Potts-model. In the case when β is large then the system prefers configurations where the particles are in the same state. When β is a large negative number then the system prefers configuration where adjacent vertices are in different states. In the limiting case β = −∞ when eβ = 0, the partition function Z counts the proper colorings of G with q colors. Formally, a map f : V ! f1; 2; : : : ; qg is a proper coloring if for all edges (x; y) 2 E we have f(x) 6= f(y). For a positive integer q let ch(G; q) denote the number of proper colorings of G with q colors. Then it turns out that ch(G; q) is a polynomial in q, and so we can study this 3 quantity even at non-integer or negative q's. This polynomial is called the chromatic polynomial [27]. It is a monic polynomial of degree v(G). Potts-model is very strongly related to the so-called random cluster model. In this model, the probability distribution is on the subsets of the edge set E(G) and for a random subset F we have 1 (F = F ) = qk(F )wjF j; P Z where k(F ) is the number of connected components of the graph G0 = (V (G);F ). Here q is non-negative and w ≥ −1, but not necessarily integers, and Z is the normalizing constant X k(F ) jF j ZRC(G; q; w) = q w : F ⊆E(G) Lemma 1.1.1. Let q be a positive integer, and eβ = 1 + w. Then ZPo(G; q; β) = ZRC(G; q; w): Proof. Clearly, 0 1 X X ZPo(G; q; β) = exp @ βI(σ(u) = σ(v))A σ:V (G)![q] (u;v)2E(G) X Y = (1 + (eβ − 1)I(σ(u) = σ(v))) σ:V (G)![q] (u;v)2E(G) X Y = (1 + wI(σ(u) = σ(v))) σ:V (G)![q] (u;v)2E(G) X X Y = wjAj I(σ(u) = σ(v)) σ:V (G)![q] A⊆E(G) (u;v)2A 0 1 X jAj X Y = w @ I(σ(u) = σ(v))A A⊆E(G) σ:V (G)![q] (u;v)2A X = wjAjqk(A) A⊆E(G) = ZRC(G; q; w): 1.1.5 Homomorphisms and partition functions It turns out that many things in the previous sections have a common generalization.