# Connections Between the Matching and Chromatic Polynomials

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1 Connections Between the Matching and Chromatic Polynomials E.J. Farrell Earl Glen Whitehead JR Presented by Doron Tiferet

2 Outline The following presentation will focus on two main results: 1. The connection between the matching and chromatic polynomials 2. A formula for the matching polynomial of a general complement of a subgraph of a graph

3 Our Scope The graphs we will relate here are all finite, undirected and contain no loops or multiple edges The two basic polynomials mentioned here are the matching polynomial and the chromatic polynomial

4 The Matching Polynomial Let G=(V,E) be a graph. A matching in G is a spanning subgraph of G, whose components are nodes and edges (two connected nodes) only Let a denote the number of matchings in G with exactly edges, and n= V. The matching polynomial is then: n 2 M G; w aw w 0 n2 1 2

5 The Matching Polynomial 2 n 2 2 M G; w a w w It can be noted that w1 and w2 are the weights (also called indeterminates) given to each node and edge, respectively. If we put 0 w w w 1 2 n 1 2, we get the following polynomial: M G; w n 2 aw 0 which is called the simple matching polynomial n

6 The Chromatic Polynomial The chromatic polynomial, noted PG ;, represent the number of colorings of the graph G with colors such that adjacent nodes receives different colors.

7 The Chromatic Polynomial 2 In this lecture, we will assume: PG; c n 0 where c is the number of ways to divide the nodes in G into n- non empty indistinguishable classes such that each class can be colored the same, and 1... n 1 n is the number of ways of choosing the colors That is called the complete graph basis n

8 Definitions Matching vector the vector of the nonzero coefficients of M G; w, written in ascending powers of w2. This vector is denoted by mg Graphs G and H are matching equivalent iff mh m G Chromatic vector the vector of the nonzero coefficients of PG ;, written in the complete graph basis. This vector is denoted by cg Graphs G and H are chromatically equivalent iff c H c G

9 Definitions 2 Graphs G and H are co-matching iff V G V H (they have the same number of nodes) and G and H are matching equivalent Graphs G and H are co-chromatic iff V G V H and G and H are chromatically equivalent Graph G is matching unique iff M G; w M H; w implies that G is isomorphic to H. Graph G is chromatically unique iff P G; P H; implies that G is isomorphic to H.

10 Basic Graphs P n - chain with n nodes K n - complete graph with n nodes K mn. - complete m by n bipartite graph c n - cycle with n nodes

11 Connection Between the Matching and Chromatic Polynomials In the following slides we will obtain the connection between the matching and the chromatic polynomials This connection will be used to determine certain qualities of the graph based on those polynomials

12 Lemma 1 Every proper coloring of a graph G with r colors induces a partition of VG into r parts such that nodes x and y belongs to the same part only if x, yv G Proof: Let us assume that nodes x and y belongs to the same part but x, yv G in this case, we get two adjacent nodes with the same color the partition wasn t made by a proper coloring

13 Lemma 2 n Let G be a graph. Then PG; a where n 0 is the number of partitions of VG into n- non-empty parts such that two nodes x and y belongs to the same part only if x, yv G Proof: notice that we need to prove that a that is, that a is also the number of the color partitions of G with exactly n- non-empty parts. c a

14 Lemma 2: proof Since each partition counted in a is also a proper color partition of G with n- parts (since there are no two adjacent nodes in the same group, we can color the nodes of the group the same), we get c a Since each proper color partition of G fulfills the condition that two adjacent nodes cannot be in the same, we also get c a This ends our proof that c a

15 Lemma 3 All color partitions of a graph G (partition of the nodes of G such that each group could be colored the same in a legal coloring) holds that all node groups contains no more than two nodes if and only if G is triangle free

16 Lemma 3: proof Suppose that G is triangle free. Let us assume that a certain group of nodes in a legal color partition of G contains more than two nodes. In this case, we now that the nodes aren t adjacent (otherwise it wouldn t be a legal coloring). Therefore, three of this nodes creates a triangle in G in contradiction to our assumption.

17 Lemma 3: proof(2) suppose that each group of nodes in a legal color partition of G contains no more than two nodes. Thus, G doesn t contain a legal coloring in which two nodes can be colored the same G doesn t contain a set of three nonadjacent nodes G is triangle free.

18 Theorem 2 n 2 n Let M G; w b w be the simple matching 0 n polynomial of graph G and PG; a be n 0 the chromatic polynomial of its complement. Then a bb for all values of. Furthermore, a b for all values of iff G is triangle free.

19 Theorem 2: proof Since b is the number of matchings with n- components, and a is the number of color partitions with n- components, we need to prove that each matching with n- components in G is counted as a color division with n- parts in G

20 Theorem 2: proof(2) Let there be such matching in G. For each component in G: if it is consisted of a single node, it is clear we can color it in a unique color in G (we can always color a node in a unique color) G G

21 Theorem 2: proof(3) If it is consisted of an edge, then the two nodes are non-adjacent in G, and therefore can be colored in the same color G G

22 Theorem 2: proof(4) Given a color partition made in this way, the corresponding matching in G is given by taing two nodes in the same group as an edge, and singleton groups as a single node. That leads us to the conclusion that each matching in G can be translated into a unique coloring of G, and therefore a b

23 Theorem 2: proof(5) We will now show that if G is triangle free, than each coloring in G translates into an unique matching in G. This will prove that a, and therefore Since G is triangle free, and according to lemma 3, each nodes group in all color partitions of G contains no more than two nodes b a b

24 Theorem 2: proof(6) Therefore we have two cases for each group in the color partition: if the group contains a single node, we will tae it as a single component in a matching in G. G G

25 Theorem 2: proof(7) Otherwise, the group must consist of two nodes. Since the nodes are in the same group, they can be colored the same. This implies that the two nodes are non-adjacent in G they are adjacent in G. We will tae the corresponding edge as a component in the matching in G. G G

26 Theorem 2: proof(8) Since this process is reversible, we get a unique matching for each coloring This proves that a b and since a b (by the first part of the theorem) we get that a for all values of We are now left to prove that this only happens if G is triangle free b

27 Theorem 2: proof(9) Let us assume G isn t triangle free, and prove that in this case a b for a certain value of The proof that a b for all values of was still valid if all color partitions of G had had no more than two nodes G G G G

28 Theorem 2: proof(10) According to lemma 3, in addition to the color partitions with less than three nodes, there is a color partition which includes a group of three (or more) nodes This color partition cannot be translated to a unique matching in G, since all the matching in G have been covered Therefore, if this color partition have l group of nodes, we get a b for l

29 Theorem 2: conclusions Let G be a graph. Then PG; M G; w' (where w means that w is replaced by ) and dually, M G; w PG; ' where ' means that is 2n n replaced with w or with w1 w2 for the general matching polynomial iff G is triangle free [theorem 1] mg c G iff G is triangle free [corollary 1.1] We can use the matching and chromatic polynomials to determine whether a graph contains triangle or not

30 Matching Polynomials of Complements of Graphs Let H be a graph and G a subgraph of G. A complement of G in H is a graph obtained from H by removing the edges of an isomorph of G If we tae H to be a labeled graph, the complement of G in H is unique, and will be denoted as G H If H K n then the complement of G in H also unique, and denoted as G n If H K mn,, the complement of G in H will be denoted as G mn,

31 Theorem 3 Let G be a graph with e edges, and let H be a labeled graph containing G as a subgraph Then: e M GH; w 1 w2 M H V S; w where is a 0 set of edges of G belonging to a matching in H ( VG denotes the group of vertices of graph G), and the summation is taen over all such matching in H, containing edges of G s

32 Theorem 3: proof We will use the principle of Inclusion and Exclusion to prove the theorem We need to find the contribution of the matchings in G that doesn t include any of the edges in G that is M G ; H w Let W() denote the contribution to M H; w of the matchings that includes at least edges of G Using the principle of Inclusion and Exclusion e we will get M GH ; w 1 W 0

33 Theorem 3: proof(2) We are left to find W() Let there be a matching which includes a certain edges of G. The weight of those edges is w 2. Since the edges are part of a matching, they cover 2 nodes of H we are left to find the donation of the matching which covers H V S. The contribution of this matching is therefore w M H V S w 2 ;

34 Theorem 3: proof(3) Since we need to find the contribution of all such matchings (which contains edges of G), we get W w2 M H V S; w w2 M H V S; w We are left to use the principle of Inclusion and Exclusion to get: e e M G ; w 1 W 1 w M H V S ; w H

35 Corollary 3.1 Let G be a graph with p nodes, and n be a positive integer such that n p p /2 p2 Also let M G; w aw1 w2 0 Then: e n; 1 2 n2; M G w a w M K w 0

36 Corollary 3.1: proof From theorem 3 we have: e M G ; w 1 w M H V S ; w H 0 2 Since in our case H K n, we get H V S Kn 2 Notice that the second summation is taen over all the matchings in K n containing edges of G. There are exactly a of those (according to the definition of the matching polynomial). The terms of the summation are equal, and therefore: e H ; n; 1 2 n2; M G w M G w a w M K w 0

37 Corollary 3.2 Let G be a bipartite graph with p nodes and p /2 p2 let M G; w aw1 w2 0 Then: e M GH ; w 1 aw2 M Km, n; w where H K mn. 0

38 Corollary 3.2: proof The proof is similar to the proof of Corollary 3.1, when we tae into consideration that H V S Km, n (because H is a complete bipartite graph), that the terms in the second summation are equal, and that there are exactly of those. a

39 Corollary 3.2: conclusions We notice that the matching polynomial of all complements of a bipartite graph G in H K mn. is identical, although isn t are co- Conclusion: all complements of G in matching Example: G H K mn. H G GH

40 Theorem 4 Let G be a graph with p nodes. p/2 e p2 ; 1 2 ; 1 2 p2; M G w a w w M G w a w M K w 0 0 Proof: This direction follows from Corollary 3.1, if we notice that G Gp n p Let us assume that e M G; w 1 aw2 M K p2; w and 0 p/2 p2 M G; w b w1 w2 0. It follows that e M G; w 1 b w2 M K p2; w 0 e e 1 b w2 M K p2; w 1 aw2 M K p2; w b a 0 0

41 Theorem 5 If graphs G and H are matching equivalent pair and F is a complete graph or a complete bipartite graph, then G F and H F are matching equivalent also Proof: G and H are matching equivalent, which means their coefficients vector is identical. According to Corollaries 3.1 and 3.2, the Coefficients vectors of G and H F F depends only in the coefficients vector of G and H and on Kn Km, n. Therefore, the coefficients vector is identical GF and H F are also matching equivalent

42 Theorem 6 Graph G is matching unique iff Proof: for every graph H which holds that M G; w M H; w we now that M G; w M H; w (according to the formula for the complementary graph theorem 4) which implies that G is isomorphic to H (from the uniqueness of G) G is isomorphic to H G is

43 Theorem 6: proof The proof follows from the first direction for every graph H, if we put G H in the first sentence we will get that graph H is matching unique if H H is. That is true for every graph H, and this completes our proof.

44 Lemma 4 Let G and H be matching equivalent graphs with p nodes and m nodes respectively. pm Then: M G; w w M H; w 1 Proof: since G and H are matching equivalent, their matching vectors are equal. Therefore: p /2 p /2 p2 m2 M G; w aw1 w2 and M H; w aw w p/2 p/2 p/2 pm pm m2 pm m2 p2 w1 M H; w w1 aw1 w2 aw1 w1 w2 aw1 w2 M G; w 0 0 0

45 Theorem 7 Let G be a graph which is matching equivalent graph H which is triangle free. * Then: PH; M G, w where w* is the transformation which replaced by w with r, where r V H V G And dually: M G, w PH ; ' where ' means that 2r2 p pr r is replaced by w1 w2

46 Theorem 7: Proof From Theorem 1, and since H is triangle free, PH; M H; w', where w means that w is replaced by r According to Lemma 4, w1 M H; w' M G; w' where r V H V G, and therefore * P H; M G; w where w* means that w is replaced by r

47 Theorem 7: Proof(2) For the dual part: from theorem 1, since H is triangle free, we get M H; w PH; ' where ' means that is replaced by 2 n n w1 w2 (where n is the number of nodes in H). r From Lemma 4: w1 M H; w M G; w where r V H V G, and therefore M G; w PH; ' where 2n n w* means that r is replaced by w1 w2

48 Theorem 8 For any tree T, mt c T T, where T is the vector of absolute values of the coefficients of the characteristic polynomial of T. Proof: Since T is a tree, it is triangle free. By corollary 1.1 (which is a conclusion of theorem 2) we get m T c T.

49 Theorem 8: Proof In the second lecture (example 18 in slide 19) we learned that if G is a tree then the characteristic and acyclic matching polynomial coincides (theorem 19, I. Gutman, 1977). Since the absolute values of the coefficients of the acyclic matching polynomial are mt we get that c T T

50 Deductions for Chromatic Equivalence and Uniqueness In the following slides we will show the connection between matching and chromatic polynomials in terms of equivalence, uniqueness, co-matchingness and cochromaticness of triangle free graphs

51 Theorem 9 Let G and H be a triangle free graphs. Then G is chromatically equivalent to H if and only if G is matching equivalent to H Proof: Since G and H are chromatically equivalent we have c G c H. From corollary 1.1 we have mg c G and mh c H (because G and H are triangle free) Therefore, mg c G c H mh G is matching equivalent to H.

52 Theorem 9: proof assuming G is matching equivalent to H, we get m G mh. From corollary 1.1 we have mg c G and mh c H (because G and H are triangle free) We now get: c G mg mh c H G is matching equivalent to H.

53 Theorem 10 Let G and H be a triangle free graphs. Then and H are co-chromatic pair iff G and H are co-matching pair Proof: V G V H and G is chromatic equivalent to H (by definition). From theorem 9 and since G and H are triangle free, G is matching equivalent to H. By the definition of the complement of a graph, we now G

54 Theorem 10: proof that V G V G and V H V H (the group of nodes stays the same). We therefore have V G V G V H V H G and H are co-matching pair. V G V H and G is matching equivalent to H (by definition). From theorem 9, G is chromatic equivalent to H. We saw that V G V G V H V H H and G are cochromatic pair

55 Theorem 11 If G is triangle free and G is chromatically unique, then there is no other graph H which is triangle free such that Proof: we will assume that there is a triangle free H such that M G, w M H, w and H G. According to theorem 2, since G and H are triangle free: M G, w PG; and M H, w PH; PG ; PH ; G H (since G is chromatically unique) G H in contradiction to our assumption

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