1 Cliques and Independent Sets Definition 1 a Set of Vertices Is Called Independent If No Two Vertices in the Set Are Adjacent

Home , Clique (graph theory), Complete bipartite graph

1 Cliques and Independent Sets

Deﬁnition 1

A set of vertices is called independent if no two vertices in the set are adjacent. A set of vertices is called a clique if every two vertices in the set are adjacent. An independent set (resp. a clique) is called maximal, if no other independent set(resp. a clique) contains it. An independent set (resp. a clique) is called maximum, if its cardi- nality is maximal among all independent sets (resp. cliques) in the graph.

α(G) (resp. ω(G)) denotes the maximum size of an independent set (resp. a clique) in a graph G; π(G) denotes the maximum size of a matching in a graph G;

1 Find a maximum independent set and a maximum clique

Problem: Given n 1 and p (1 p n), construct a graph with n vertices ≥ ≤ ≤ and clique-size p, which contains the maximum number of edges. ≤ Dual Problem: Given n 1 and p (1 p n), construct an n-vertex graph with ≥ ≤ ≤ the minimal number of edges for which the size of any independent set is p. ≤ Special Case: (p = 2) Find the maximum number of edges in a graph with n vertices and without triangles.

2 Theorem 1 (a) Every triangle-free graph with 2k vertices has at most k2 edges. (b) The only triangle-free graph with 2k vertices and k2 edges is the complete bipartite graph with the partitions of sizes k each.

Proof. Induction on k. PART (a). Base. The result is straightforward for k = 1. Inductive step. Let the statement be correct for all triangle-free graphs with 2k 2 vertices, and let G be a triangle-free graph with − 2k vertices. Select an edge (a, b) E(G). The set of all edges of G ∈ consists of (i) the edges in G a, b ; − { } (ii) the edges connecting a, b with the rest of G; and { } (iii) the edge (a, b) itself. By induction, E(G a, b ) (k 1)2. | − { } |≤ − Furthermore, there are at most 2k 2 edges of type (ii). (Explain − why)

2k−2 vertices; no triangles

e(G) e(G a, b )+2k 2+1 ≤ − { } − (k 1)2 +2k 1= k2 2k +1+2k 1= k2. ≤ − − − − 3 PART (b). Denote T (2k, 2) the complete bipartite graph on 2k vertices with the partitions of sizes k each. It is easy to see that for k = 1, the only triangle-free graph with k2 edges is T (2k, 2). Assume, inductively, that the second part of the Theorem holds for all graphs with 2k 2 vertices, and let G bea2k-vertex triangle-free − graph with k2 edges. Then, as before consider G a, b for some − { } edge ab and analyze the inequality e(G) e(G a, b )+2k 2+1 (k 1)2 +2k 1= k2. ≤ − { } − ≤ − − The following is obvious: for this inequality to be an equality, G − a, b must have exactly (k 1)2 edges and the number of edges { } − connecting a, b with the rest must be equal to 2k 2. { } − The ﬁrst requirement implies, inductively, that G a, b = T (2k 2, 2), − { } − which in turn, implies that a (resp. b) is adjacent to the vertices of one part of G a, b only. Those parts must be distinct which − { } proves that G = T (2k, 2).

4 Comment. The Theorem above can be expanded to graphs with an arbitrary number n of vertices (even or odd):

Theorem 2 2 (a) Every triangle-free graph with n vertices has at most n ⌊ 4 ⌋ edges. 2 (b) The only triangle-free graph with n vertices and n edges ⌊ 4 ⌋ is the complete bipartite graph with the partitions of sizes n ⌊ 2 ⌋ and n+1 . ⌊ 2 ⌋

5 Deﬁnition 2

Given n and p, Tur´an’s graph T (n,p) obtained by partitioning n vertices into p disjoint sets of almost the same size (within 1) and setting edges to be all pairs comprised of vertices from diﬀerent partitions. The sizes of the partitions of T (n,p) are obtained by dividing n by p with a remainder: n = p q + r, where 0 r p 1. × ≤ ≤ − The r partitions of T (n,p) are of size q +1, and the remaining p r − partitions are of size q. It is easy to prove the following

Lemma 1 If n p, then T (n,p) is a complete graph on n ver- ≤ tices. For any n 2 and 1 p

Theorem 3 (Tur´an[1944]) Given positive integers n and p, the number of edges of any graph with n vertices and without a clique of size p +1 is at most e(T (n,p)).

6 Theorem 4 (Tur´an[1944])

Every graph of order n and size m contains an independent set of size n2/(2m + n). ≥ Proof. We present a greedy algorithm which constructs an independent set whose size is n2/(2m + n). ≥

I = ; ∅ H = G; while (not done) select a vertex v V (H) of the minimal degree in H; ∈ I = I v ; ∪ { } H = H v all the vertices adjacent to v ; − { }−{ } To prove that the algorithm constructs an independent set of size n2/(2m + n), we use induction on n. The statement is obviously ≥ true for the 1-vertex graph. Suppose, the theorem holds for every graph with

8 Problem 1 Determine the disconnected n-vertex graphs (n 2) ≥ that have the maximum number of edges.

Problem 2 Determine the maximum number of edges in an n- vertex graph (without parallel edges) that has an independent set of size α.

Problem 3 Let G be a simple graph with n 4 vertices. Prove ≥ that if G has more than n2/4 edges, then it has a vertex whose deletion leaves a graph with more than (n 1)2/4 edges. − Problem 4 Prove that every n-vertex triangle-free simple graph with the maximum number of edges is isomorphic to K n/2 , n/2 . ⌊ ⌋ ⌈ ⌉

Problem 5 A ﬂat circular city of radius six miles is patrolled by eighteen police cars, which communicate with one another by radio. If the range of a radio is nine miles, show that at any time, there is always at least two cars each of which can communicate with at least ﬁve other cars.

9 2 Dominating sets

Deﬁnition 3

For a graph G, a set D V (G) of vertices is called dominating if ⊆ NG(D) = V (G), that is if every vertex in V (G) is either in D or adjacent to a veretex in D. A dominating set is called minimal if no subsets of D is dominating. A dominating set is called minimum if no smaller set in G is dominating.

γ(G) denotes the minumal size of a dominating set in a graph G.

Lower and upper bounds for γ(G). Theorem. For a simple graph G, let α(G) denote the maximal size of an independent set, and let diam(G) denote the diameter of G. Then diam(G)+1 γ(G) α(G). ⌈ 3 ⌉≤ ≤

10 3 Discrete Mathematics revisited.

Facts to remember

Given set X, the number of subsets of X is given by • X X 2 =2| |. | | The number of all permutations of a set X with n elements is • nn n!= n (n 1) (n 2) ... 2 1 √2πn. × − × − × × × ≈ en The number n of k-subsets of a set X with n elements • k n n!   = . k k!(n k)!   −

n+1/2 • v  n  nu 2 2 2 u = .   u n/2 ≈ tπn √πn

11 4 Ramsey Theory

The Simplest Ramsey Type Theorem. In any collection of six people either three of them mutually know each other or three of them mutually do not know each other.

Theorem 5 (Ramsey [1930])

For every two positive integers k and l, there exists a smallest integer R(k,l) such that every graph of order R(k,l) contains either a clique on k vertices or an independent set on l vertices. Proof (Erd˝os and Szekeres [1935]). By induction on k and l. The statement is correct if k =2 or if l = 2: R(2,l)= l and R(k, 2) = k.

Let k,l 3 and assume, inductively, that the existence of R(k 1,l) ≥ − and R(k,l 1) has been established. We will prove that − R(k,l) R(k 1,l)+ R(k,l 1). ≤ − − Let G be a graph with n = n(G)= R(k 1,l)+ R(k,l 1) vertices − − and let v be an arbitrary vertex of G. Denote N(v) and N(v) the set of vertices adjacent to v and the set of vertices that are not adjacent to v, respectively. Then either N(v) R(k 1,l) or N(v) R(k,l 1). | |≥ − | |≥ − 12 Case of N(v) R(k 1,l). | |≥ − By induction, N(v) must contain either a clique of size k 1, or an − independent set of size l. In the former case, the clique and v yield a clique of size k in G; in the latter case, the same independent set is is an independent set of size l. Case of N(v) R(k,l 1). | |≥ − By induction, N(v) must contain either a clique of size k, or an independent set of size l 1. In the former case, an independent set − and v yield an independent set of size k in G, in the latter case, the same set of size l is a clique of size l in G.

13 Deﬁnition 1: The Ramsey number R(p, q) is the smallest integer n such that in any 2-coloring of the edges of a complete graph on n vertices, Kn, by red and blue, there is either a red Kp (i.e. a complete subgraph on p vertices all of whose edges are colored red) or a blue Kq.

Corollary 1. For all p, q 1, ≥ R(p, q) R(p, q 1) + R(p 1, q). ≤ − − If both R(p, q 1) and R(p 1, q) are even, then − − R(p, q)

1 0 1 13 2 2 16 12 3 15 3 4 11 14

4 5 10 13 6 5 12 9 7 11 6 8 8 10 7 9

43 R(5, 5) 49; 35 R(4.6) 41; 102 R(6, 6) 165. ≤ ≤ ≤ ≤ ≤ ≤

14 Theorem 6 Erd˝os and Szekeres [1935] R(p,p) > 2p/2 for all p 3. ≥ Proof (1). Consider the set of all graphs on 1, 2,...,n . There n n { p } (2) (2) (2) are 2 of them. Each clique occurs in 2 − of these graphs. n p (2) (2) Similarly, there are 2 − graphs for which a particular set occurs n p (2) (2)+1 as independent set. Together, there are 2 − graphs for which n a particular set is either independent or a clique. Since there are p n n p (2) n (2) (2)+1 subsets of size p, out of 2 graphs at most p2 − contain either a clique of size p or an independent set of size p. If n< 2p/2, n p n n (2) (2)+1 (2) then p2 − < 2 , which proves the theorem.

Proof (2). Consider a random coloring of Kn, where each edge is colored independently and with the same probability in either red or blue. For a ﬁxed set D of p vertices, let AD be the event that the induced subgraph of Kn on D is monochromatic; denote the probability of AD by Pr(DA). To compute Pr(DA), note that the event happens iﬀ the colors of p 1 edges coincide with that of the 2 − remaining edge. Thus, p p ((2) 1) 1 (2) Pr[DA]=2− − =2 − . n Since there are p possible choices for A, the probability that at least one of these events occurs is at most

p n 1 (2) 2 − .   p If n is selected so that

p n 1 (2) 2 − < 1, ( )   p ∗

15 there is a positive probability that no event DA occurs, and there is a 2-coloring of Kn without a monochromatic Kp, that is R(p,p) >n. To ﬁnish the proof, we show that any n 2p/2 would satisfy (*). ≤ Indeed p p (p 2)/2 n 1 (2) n 2 − 2 − < p < 1   (2) 1 p p!2 − ≤ p! for all p> 3.

16 Theorem 7 (Ramsey [1930])

For every two positive integers p and q, (a) there exists a ﬁnite Ramsey number R(p, q)= R(Kp, Kq). (b) R(p, q) R(p 1, q)+ R(p, q 1); and ≤ − − (c) if both R(p 1, q) and R(p, q 1) are even, then − − R(p, q)

17 Examples. From theorem 1, R(2, q) = q; R(p, 2) = p; and R(3, 3)=6, we compute

R(3, 4) R(2, 4) + R(3, 3)=4+6 thm= 3 R(3, 4) 9. ≤ ⇒ ≤

R(3, 5) R(2, 5) + R(3, 4)=5+9=14. ≤

R(3, 6) R(2, 6) + R(3, 5)=6+14 thm= 3 R(3, 6) 19. ≤ ⇒ ≤

R(3, 7) R(2, 7) + R(3, 6)=7+19=26. ≤

R(3, 8) R(2, 8) + R(3, 7)=8+26 thm= 3 R(3, 8) 33. ≤ ⇒ ≤ R(p, q) 3 4 5 3 6 9 14 4 9 18 31 5 14 31 62

18 Theorem 8 For every integer n 3, ≥ n2 +3 R(3,n) . ≤ 2 Proof. Induction on n. 2 3 +3 Base. n = 3. R(3, 3)=6= 2 . Inductive step. Assume R(3,n 1) (n 1)2 + 3)/2 for some − ≤ − n> 3, and consider R(3,n). By theorem 1, R(3,n) R(2,n)+ R(3,n 1) = n + R(3,n 1). ≤ − − Inductively, (n 1)2 +3 R(3,n 1) = − . − 2 Then R(3,n) R(2,n)+ R(3,n 1) ≤ − (n 1)2 +3 n2 +4 n + − = ≤ 2 2

To complete the proof, we need to show that the last inequality is strict. This is obvious, if n is odd. It is also true if (n 1)2 +3 R(3,n 1) < − . − 2 Thus, let n = 2k for some integer k and let R(3,n 1) = ((n − − 1)2 + 3)/2. Then, (n 1)2 +3 4k2 4k +1+3 R(3,n 1) = − = − =2k2 2k +2. − 2 2 − Since, R(2,n) and R(3,n 1) are both even, theorem 3 applies − yielding the result.

19 5 Ramsey Theory (generalizations)

Deﬁnition 4 Given graphs G1,...,Gk, the Ramsey number R(G1,...,Gk) is the smallest integer n such that for every k- coloring of the edges E(Kn), G contains a copy of Gi for some i = 1,...,k all of whose edges are of the same color (monochromatic copy).

Theorem 9 (Chv´atal [1977]) If T is a p-vertex tree, then R(T, Kq)= (p 1)(q 1)+1. − −

The Chvatal graph for 6-tree and 4-clique

Deﬁnition 5

Given graphs G and H without common vertices, G + H denotes the graph R with V (R)= V (G) V (H) and E(R)= E(G) E(H). ∪ ∪ For an integer m> 0 and a graph G, mG denotes G + ...G . m | {z }

Theorem 10 (Burr-Erd˝os-Spencer [1975]) For m> 1, R(mK3, mK3)= 5m.

20 IK2m-1 3m-1

The Burr-Erdos-Spencer graph for R(mK,mK ) 3 3

21 Problem 6 Show that (a) R(5, 5) 70; ≤ (b) R(5, 6) 126. ≤

Problem 7 Show that if the edges of the complete graph are colored red, white, blue, green, brown, and purple, then if there are suﬃciently many vertices, then there is a 4-gon all of whose edges are colored the same color.

Problem 8 Show that if the edges of the complete graph are colored red, white, blue, and green, then if there are suﬃciently many vertices, and there is no red, white, or blue triangle, then there is a complete 12-gon all of whose edges are colored green