Bipartite Subgraphs of Integer Weighted Graphs
|
|
- Silas King
- 5 years ago
- Views:
Transcription
1 Bipartite Subgraphs of Integer Weighted Graphs Noga Alon Eran Halperin February, 00 Abstract For every integer p > 0 let f(p be the minimum possible value of the maximum weight of a cut in an integer weighted graph with total weight p. It is shown that for every large n and every m < n, f( n + m = n + min( n, f(m. This supplies the precise value of f(p for many values of p including, e.g, all p = ( n + ( m when n is large enough and m n. 1 Introduction All graphs in this paper contain no loops. maximum number of edges in a bipartite subgraph of G. For a simple graph G = (V, E, let f(g denote the For every p > 0, let g(p denote the minimum value of f(g, as G ranges over all simple graphs with p edges. Thus, g(p is the largest integer such that any simple graph with p edges contains a bipartite subgraph with at least g(p edges. There are several papers providing bounds for g(p, see, e.g, [1]-[5]. Edwards [],[3] proved that for every p g(p p p Equality holds for all complete graphs. Alon [1] showed that there is a positive constant C so that g(p p + p 8 + C p1/, for all p. He also proved that there is a positive constant c and infinitely many positive integers p satisfying g(p p + p 8 + c p1/. Department of Mathematics, School of Mathematical Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel. address : noga@math.tau.ac.il. Department of Computer Science, School of Mathematical Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel. address : heran@math.tau.ac.il. Final version: to appear in Discrete Mathematics. 1
2 We raise the following conjecture: Conjecture 1.1 For every n and for every 0 m < n, g( ( n + m = n + min( n, g(m. If Conjecture 1.1 is true then given p, we can construct an extremal graph G = (V, E with p edges for which f(g = g(p as follows. Let n 1 be the greatest integer such that ( n 1 p and let p 1 = p ( n 1. For every i > 1 define, by induction, ni to be the greatest integer such that ( ni pi 1 if p i 1 > 0 and n i = 0 otherwise and let p i = p i 1 ( n i. Let k be an integer such that n 1 + n n k 1 + n k + n k is minimal. If Conjecture 1.1 is true then g(p = n 1 + n n k 1 + n k + n k. For every 1 i < k let G i be the complete graph on n i vertices. Let G k be an arbitrary simple graph on n k + 1 vertices with p k 1 edges. If Conjecture 1.1 is true, then an extremal graph G is obtained by taking the disjoint union of G 1, G,..., G k. An integer weighted graph is a graph G = (V, E in which each edge has a (not necessary positive integral weight. For an integer weighted graph G = (V, E, let f(g denote the maximum total weight in a bipartite subgraph of G. For every p > 0, let f(p denote the minimum value of f(g, as G ranges over all integer weighted graphs with total weight p. Thus, f(p is the largest integer such that any integer weighted graph with total weight p contains a bipartite subgraph with total weight no less than f(p. The following conjecture is stronger than Conjecture 1.1: Conjecture 1. for every n and for every 0 m < n f( ( n + m = n + min( n, f(m. If Conjecture 1. is true then so is Conjecture 1.1 and this implies that f(p = g(p for every p > 0. Therefore, we consider the function f here and prove the following theorem: Theorem 1.3 Let G = (V, E be an integer weighted graph with total weight ( n + m, where 0 m < n. Then f(g n + min( n, f(m, provided n is sufficiently large. Therefore, f( ( n + m = n + min( n, f(m. In Section we give an outline of the proof of Theorem 1.3, and Section 3 describes the complete proof of the theorem. An Outline of the Proof It is obvious that f( ( n + m n + min( n, f(m because we can construct integer weighted graphs implying the inequality. Thus it suffices to prove the first part of the theorem, i.e, to prove that given an integer weighted graph G = (V, E with weight function w and with total weight ( n + m, f(g n + min( n n, f(m. We assume that f(g < + min( n, f(m and get a contradiction.
3 To do so, we first observe that nonpositive weighted edges can be contracted, and therefore we can assume that V n and that for every u, v V, w(u, v > 0. Next we show in Corollary 3. and Lemma 3.3 that there is a small set U V such that for every u, v V \ U we have w(u, v = 1. We view the graph G as a complete graph with another graph on top of it (the multiple edges, where every vertex of U represents a set of vertices in this virtual graph which are forced to be in the same side in every bipartite graph that we take, while every vertex of V \ U represents one vertex in the virtual graph. If v i U and the average weight of the edges (v, v i where v V \ U is d i, then v i represents a set of d i vertices in this virtual graph. We next show (see Lemma 3.6 that in this way the number of represented vertices is at least n, since otherwise the total weight of the edges is less then ( n + m, contradicting the assumption. Next, it is shown that there are two possible cases. The first is that most of the vertices behave as we view them, i.e for most of the vertices of V \ U the weight of the edges between them and a vertex of U is the average weight of the edges between that vertex and V \U. In this case, it is shown in Corollary 3.9 that the number of represented vertices is at most n and thus is exactly n. We take the graph which is on top of the complete graph and find a large weighted bipartite subgraph in this graph and then we add the vertices which are not in this graph and get, using the induction hypothesis, a bipartite subgraph with sufficiently large total weight. In the second case (see Lemma 3.7 and Lemma 3.8 it is shown that w(u, V \ U is very large. In this case we take the n vertices of V \ U connected to U by the heaviest edges and put them in one side and all the other vertices in the other side. As a result of the fact that the total weight w(u, V \ U of edges between U and V \ U is very large, we get a bipartite subgraph of sufficiently large weight. 3 The proof of the main Theorem We start with some definitions. Let G = (V, E be an integer weighted graph with weight function w : E Z. Let r be defined by the following equation: f(m = m + r. It is easy and known (see [1] that g(m m + m 8 + C m 1 where C is an absolute constant. Since f(m g(m, r m 8 + C m 1. For every two sets of vertices U, W let w(u, W denote the total weight of the edges with one end in U and the other end in W, i.e w(u, W = u U,w W w(u, v. If U or W are singletons we write the vertex they contain instead of the corresponding sets. Define a 3
4 multimatching M of G to be a partition of the vertices of G into pairs except for maybe one vertex if V is odd. If v, u V and (v, u M we say that v is matched with u by M. The value of the multimatching M is defined to be : M = (v,u M (w(v, u 1 A maximum multimatching is a multimatching of maximum value. It is obvious that n f( + m n + min( n, f(m because we can actually construct integer weighted graphs implying the inequality. Therefore it suffices to prove that given an integer weighted graph G = (V, E with weight function w and with total weight ( n + m, f(g n + min( n, f(m. Assume G has total weight ( n +m and f(g < n +min( n, f(m. We assume that for every v, u V w(v, u > 0, since otherwise we could define a new graph G by contracting the vertices v, u into one vertex v, thus getting that the total weight of G is not less than the total weight of G. We can reduce weight from G to get a graph G with total weight equal to that of G and with f(g f(g. Thus, we assume without loss of generality that G is a multigraph such that any two vertices of G are connected by at least one edge. In such a multigraph with p edges it is known (see [] that f(g p + p 8 O(1. Therefore, r m 8 O(1 and thus r is m (1 + o(1. Suppose then that V = {v 1, v,..., v n k } where k 0 by the above observations. We need a lemma about multimatchings: Lemma 3.1 Let H = (V, E be a multigraph with w(u, v > 0 for every u, v V and let M be a maximum multimatching in H. If V is even then E ( V 1 ( M + V E V ( M + ( V 1.. If V is odd then Proof: First, assume that V is even. Let (u 1, w 1,..., (u l, w l be the pairs in the multimatching M, where l = V. for every i j we have because of the maximality of M. w({u i, w i }, {u j, w j } (w(u i, w i + w(u j, w j So, if we sum up over all 1 i j t we have E ( (l ( M + l = ( V 1 ( M + V. If V is odd we can add a vertex u to V and add an edge (u, v for every v V. So, by the first part of the lemma, the number of edges V +1 V +1 V 1 in the new graph is bounded by ( V ( M +, so E V ( M + V = V ( M + Let M be a maximum multimatching in G. We next prove two simple bounds on the value of M.
5 Corollary 3. M k. Proof: Thus, M k. By Lemma 3.1 we have: M + n k E n k = m + ( k ( + n k + k (n k n k + k 1 n k Lemma 3.3 M k+r. Proof: Assume M > k+r. If we take the pairs of M in a greedy way we get: f(g E + M + n k 1 = n + m k M n + f(m By Corollary 3. and Lemma 3.3 we have the following corollary: Corollary 3. k r. Let a := M k, then M = k+a, and by Lemma 3.3 and Corollary 3. k a r. Let U = {v V u, (v, u M and w(v, u > 1}. Let t denote the size of U. We have t M = k+a. We assume without loss of generality that U = {v 1,..., v t }. For every 1 i t define h i and d i to be the integers such that w(v i, V \ U = d i V \ U + h i where h i V \U V \U (in case h i = let h i be positive. Furthermore, let x i denote the number of vertices v V \ U such that w(v, v i d i. Lemma 3.5 Let v i U. Let v j be the matched vertex of v i in M. Then max( h i, x i r a + w(v i, v j. Proof: Without loss of generality, w(v i, v t+1 w(v i, v t+... w(v i, v n k. Let p := r a + w(v i, v j. We show that max( h i, x i p. Let A 1 = {v t+1,..., v t+p+1 }. Let A = {v n k p,..., v n k }. Let B 1 = A 1 and B = A {v i } and add v j to B 1 or to B with equal probability. Now, the expected value of w(b 1, B is: w(a 1, A + w(v i, A 1 + w(v j, {v i } A 1 A As A 1, A V \ U, the subgraph of G induced on A 1 A is a complete graph on p + vertices. Thus, w(a 1, A = w(a 1 A + A 1. Furthermore, w(v i, A 1 = w(v i,a 1 A + w(v i,a 1 w(v i,a and w(a 1 A {v i, v j } = w(a 1 A + w(v i, A 1 A + w(v j, {v i } A 1 A. Therefore, the expected value of w(b 1, B is: w(a 1 A + A 1 + w(v i, A 1 A + w(v i, A 1 w(v i, A + w(v j, {v i } A 1 A = w(a 1 A {v i, v j } + A 1 + (w(v i, A 1 w(v i, A 5
6 If w(v t+p+1, v i > w(v n k p, v i then w(v i, A 1 w(v i, A p + 1. see that w(v i, A 1 w(v i, A max( h i, x i. w(v i, A 1 w(v i, A p + 1 in both cases. Otherwise it is easy to Now, if the lemma was false then we have If we continue to join in a greedy way the other pairs of M trying to maximize the weight in the cut we get a bipartite graph of size at least E + A 1 + w(v i, A 1 w(v i, A + n k A 1 A 1 + M (w(v i, v j 1 n + m 1 + p a w(v i, v j contradiction. n + m + r, Lemma 3.6 t (d i 1 k. Proof: Assume that t (d i 1 k 1. By Lemma 3.1 we have w(u ( t 1 ( M + t (1 also, w(v \ U = n k t By the definition of d i and h i we know that w(v \ U, U = t (d i V \ U + h i. Now, ( w(v \ U, U = = t ((d i 1 V \ U + h i + t V \ U t V \ U (d i 1 + h i + t V \ U t t (k 1 (n k t + h i + t (n k t It is easy to see that by Lemma 3.5 we have t h i M + t + (r a t By the last inequality and by equations (1 and ( we get that E n k t ( t 1 ( M + t + + (k 1 (n k t + 6
7 + M + t + (r a t + t (n k t n k t t = ( + t (n k t (k 1(n k + M + t ( M k + + r a n k = + (k 1 (n k + M + t ( r a + k + M n k = + (k 1 (n k + k + a + t ( r k + n k + (k 1 (n k + k + r + (k + r ( r k + k E m n + 7 k + 5 r + r k If k = then the last expression attains its maximum value which is E m n r r + < E The last inequality follows from the fact that n is large enough, m < n and r = m (1 + o(1. Lemma 3.7 k a. Proof: Assume the opposite, i.e k > a. By Lemma 3.1 we have w(u ( t 1 ( M + t = t (k + a k+a + ( t (. Also, we have w(v \ U = n k t. Hence, w(u, V \ U = = E w(v \ U w(u n k t E (t (k + a k + a t + = (n k t( t + k + t k + = (n k t( t + k + t (k a + k + m t (k + a + k + a k + m + k + a As a k and t k+a we get that t (k a k a.combining with the last inequality we get: Now, if k > a then it follows that w(u, V \ U (n k t( t + k + m + k a w(u, V \ U (n k t ( t + k + m (3 7
8 Without loss of generality, U = {v 1,..., v t } and w(v t+1, U w(v t+, U... w(v n k, U. Let A = {v t+1,..., v t+ n }. If w(v t+ n, U t + k + 1 we get that w(v t+i, V \ A n + 1 for every 1 i n, and then f(g w(a, V \ A n ( n + 1 = n + n Otherwise, for every n + 1 i n k t we have w(v t+i, U t + k and therefore w(v \ (U A, U ( t + k V \ (U A = ( t + k ( n t k. Thus, by this observation and inequality (3 we get: f(g w(a, V \ A = w(a, V \ (U A + w(v \ U, U w(v \ (U A, U n ( n t k + (n k t ( t + k + m (( t + k ( n t k = m + n n n + f(m Lemma 3.8 t x i n k t. Proof: For every 1 i t let u i be the matched vertex of v i. Using Lemma 3.5 we get: t k + t + x i t k + t + ( r a + w(v i, u i = k + t + t (r a + M As a result of this and the fact that M = k+a and t M we have: t k + t + x i k + 3 a + (k + a (r a By Lemma 3.7 and by inequality (3 we get that k + t + t x i ( + a + a (1+ 1 (r a. If a = r + then the expression ( + a + a (1+ 1 (r a attains its maximum value which is r r ( (1 + 1 which is at most n, (since n is sufficiently large. As a result of the lemma we have the following corollary: 8
9 Corollary 3.9 t (d i 1 k. Proof: Let A = {v V \ U w(v, v i = d i for 1 i t}. By Lemma 3.8, A n. Let B A with B = n. Then, t f(g w(b, V \ B = ( (d i 1 + n k n Thus, if t (d i 1 > k then G is not a counter example to the theorem. We are now ready to prove the main theorem. For every v i U put d vi = d i. For every v V \ U let d v = 1. Note that by Corollary 3.9 and by Lemma 3.6 we have v V d v = n. For every A V define d A = v A d v. For every A, B V let C(A, B = w(a, B d A d B. By Corollary 3.9 and Lemma 3.6 we have u,v V C(u, v = E u,v V (d v d u m. Let B = {v V u V s.t C(u, v 0} U. By Lemma 3.8 we have V \ B n. Let H be the subgraph of G induced on B. We have u,v B C(u, v = u,v V C(u, v m. H with the weight function C is an integer weighted graph and therefore f(h f(m. Thus H contains a bipartite subgraph with total weight at least f(m, i.e there are two disjoint sets B 1, B B with C(B 1, B f(m. Now, w(b 1, B = C(B 1, B + d B1 d B f(m + d B1 d B. Let T be the union of B 1 and ( n d B 1 vertices of V \ B. Then, f(g w(t, V \ T = = w(b 1, B + ( n d B 1 d B + ( n d B d B1 + ( n d B 1 ( n d B = w(b 1, B + n d B 1 d B n + f(m Concluding Remarks The proof of the theorem holds only for large n. It seems plausible that its assertion holds also for smaller n. Nevertheless, Theorem 1.3 gives us the ability to find the exact values of f(p for infinitely many values of p. In all these cases we can even find the exact value of g(p because g(p f(p and we can find a simple graph G with p edges and with f(g = f(p, concluding that f(p g(p f(g = f(p. The following proposition, illustrating this reasoning, is a consequence of Theorem 1.3 and the fact (proved in [], [] that g( ( n = n for every n : 9
10 Proposition.1 For every sufficiently large n and for all s satisfying s n, we have f( ( n ( + s ( = g( n ( + s = n + s. We can also deduce from Theorem 1.3: Proposition. For all sufficiently large n and for all m < n satisfying m m+1 8 > n we have f( ( n ( + m = g( n + m = (n+1. It would be nice to prove Conjecture 1.1 and Conjecture 1. for all n. Another fact which is a consequence of the proof of Theorem 1.3 is that if we define a function h(p as we did for g and f, but where the minimum is taken over all multigraphs, then by the observation in the beginning of the proof of Theorem 1.3 we get that g(p = h(p for every p 1. References [1] N. Alon, Bipartite subgraphs, Combinatorica 16 (1996, [] C.S.Edwards Some extremal properties of bipartite subgraphs Canadian Journal of Mathematics 3 (1973, [3] C.S.Edwards An improved lower bound for the number of edges in a largest bipartite subgraph Proc. nd Czechoslovak Symposium on Graph Theory, Prague, (1975, [] P. Erdős, A. Gyárfás and Y. Kohayakawa, The size of the largest bipartite subgraph, to appear in Discrete Mathematics. [5] S. Poljak and Zs. Tuza, The max cut problem - a survey, In: DIMACS Series in Discrete Mathematics, (W. Cook, L. Lovász and P. D. Seymour, eds., AMS Press, to appear. 10
Independent Transversals in r-partite Graphs
Independent Transversals in r-partite Graphs Raphael Yuster Department of Mathematics Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv, Israel Abstract Let G(r, n) denote
More informationProperly colored Hamilton cycles in edge colored complete graphs
Properly colored Hamilton cycles in edge colored complete graphs N. Alon G. Gutin Dedicated to the memory of Paul Erdős Abstract It is shown that for every ɛ > 0 and n > n 0 (ɛ), any complete graph K on
More informationChoosability and fractional chromatic numbers
Choosability and fractional chromatic numbers Noga Alon Zs. Tuza M. Voigt This copy was printed on October 11, 1995 Abstract A graph G is (a, b)-choosable if for any assignment of a list of a colors to
More informationT -choosability in graphs
T -choosability in graphs Noga Alon 1 Department of Mathematics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel. and Ayal Zaks 2 Department of Statistics and
More informationA lattice point problem and additive number theory
A lattice point problem and additive number theory Noga Alon and Moshe Dubiner Department of Mathematics Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv, Israel Abstract
More informationA Separator Theorem for Graphs with an Excluded Minor and its Applications
A Separator Theorem for Graphs with an Excluded Minor and its Applications Noga Alon IBM Almaden Research Center, San Jose, CA 95120,USA and Sackler Faculty of Exact Sciences, Tel Aviv University, Tel
More informationEven Cycles in Hypergraphs.
Even Cycles in Hypergraphs. Alexandr Kostochka Jacques Verstraëte Abstract A cycle in a hypergraph A is an alternating cyclic sequence A 0, v 0, A 1, v 1,..., A k 1, v k 1, A 0 of distinct edges A i and
More informationThe concentration of the chromatic number of random graphs
The concentration of the chromatic number of random graphs Noga Alon Michael Krivelevich Abstract We prove that for every constant δ > 0 the chromatic number of the random graph G(n, p) with p = n 1/2
More informationOn judicious bipartitions of graphs
On judicious bipartitions of graphs Jie Ma Xingxing Yu Abstract For a positive integer m, let fm be the maximum value t such that any graph with m edges has a bipartite subgraph of size at least t, and
More informationThe Turán number of sparse spanning graphs
The Turán number of sparse spanning graphs Noga Alon Raphael Yuster Abstract For a graph H, the extremal number ex(n, H) is the maximum number of edges in a graph of order n not containing a subgraph isomorphic
More informationBetter bounds for k-partitions of graphs
Better bounds for -partitions of graphs Baogang Xu School of Mathematics, Nanjing Normal University 1 Wenyuan Road, Yadong New District, Nanjing, 1006, China Email: baogxu@njnu.edu.cn Xingxing Yu School
More informationIndependence numbers of locally sparse graphs and a Ramsey type problem
Independence numbers of locally sparse graphs and a Ramsey type problem Noga Alon Abstract Let G = (V, E) be a graph on n vertices with average degree t 1 in which for every vertex v V the induced subgraph
More informationMINIMALLY NON-PFAFFIAN GRAPHS
MINIMALLY NON-PFAFFIAN GRAPHS SERGUEI NORINE AND ROBIN THOMAS Abstract. We consider the question of characterizing Pfaffian graphs. We exhibit an infinite family of non-pfaffian graphs minimal with respect
More informationObservation 4.1 G has a proper separation of order 0 if and only if G is disconnected.
4 Connectivity 2-connectivity Separation: A separation of G of order k is a pair of subgraphs (H, K) with H K = G and E(H K) = and V (H) V (K) = k. Such a separation is proper if V (H) \ V (K) and V (K)
More informationObservation 4.1 G has a proper separation of order 0 if and only if G is disconnected.
4 Connectivity 2-connectivity Separation: A separation of G of order k is a pair of subgraphs (H 1, H 2 ) so that H 1 H 2 = G E(H 1 ) E(H 2 ) = V (H 1 ) V (H 2 ) = k Such a separation is proper if V (H
More informationGraph coloring, perfect graphs
Lecture 5 (05.04.2013) Graph coloring, perfect graphs Scribe: Tomasz Kociumaka Lecturer: Marcin Pilipczuk 1 Introduction to graph coloring Definition 1. Let G be a simple undirected graph and k a positive
More informationThe Algorithmic Aspects of the Regularity Lemma
The Algorithmic Aspects of the Regularity Lemma N. Alon R. A. Duke H. Lefmann V. Rödl R. Yuster Abstract The Regularity Lemma of Szemerédi is a result that asserts that every graph can be partitioned in
More informationK 4 -free graphs with no odd holes
K 4 -free graphs with no odd holes Maria Chudnovsky 1 Columbia University, New York NY 10027 Neil Robertson 2 Ohio State University, Columbus, Ohio 43210 Paul Seymour 3 Princeton University, Princeton
More informationInduced subgraphs of prescribed size
Induced subgraphs of prescribed size Noga Alon Michael Krivelevich Benny Sudakov Abstract A subgraph of a graph G is called trivial if it is either a clique or an independent set. Let q(g denote the maximum
More informationThe edge-density for K 2,t minors
The edge-density for K,t minors Maria Chudnovsky 1 Columbia University, New York, NY 1007 Bruce Reed McGill University, Montreal, QC Paul Seymour Princeton University, Princeton, NJ 08544 December 5 007;
More informationBalanced bipartitions of graphs
2010.7 - Dedicated to Professor Feng Tian on the occasion of his 70th birthday Balanced bipartitions of graphs Baogang Xu School of Mathematical Science, Nanjing Normal University baogxu@njnu.edu.cn or
More informationThe 123 Theorem and its extensions
The 123 Theorem and its extensions Noga Alon and Raphael Yuster Department of Mathematics Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv, Israel Abstract It is shown
More informationA note on network reliability
A note on network reliability Noga Alon Institute for Advanced Study, Princeton, NJ 08540 and Department of Mathematics Tel Aviv University, Tel Aviv, Israel Let G = (V, E) be a loopless undirected multigraph,
More informationTesting Equality in Communication Graphs
Electronic Colloquium on Computational Complexity, Report No. 86 (2016) Testing Equality in Communication Graphs Noga Alon Klim Efremenko Benny Sudakov Abstract Let G = (V, E) be a connected undirected
More informationBounds for pairs in partitions of graphs
Bounds for pairs in partitions of graphs Jie Ma Xingxing Yu School of Mathematics Georgia Institute of Technology Atlanta, GA 30332-0160, USA Abstract In this paper we study the following problem of Bollobás
More informationPacking and Covering Dense Graphs
Packing and Covering Dense Graphs Noga Alon Yair Caro Raphael Yuster Abstract Let d be a positive integer. A graph G is called d-divisible if d divides the degree of each vertex of G. G is called nowhere
More informationMINIMALLY NON-PFAFFIAN GRAPHS
MINIMALLY NON-PFAFFIAN GRAPHS SERGUEI NORINE AND ROBIN THOMAS Abstract. We consider the question of characterizing Pfaffian graphs. We exhibit an infinite family of non-pfaffian graphs minimal with respect
More informationNeighborly families of boxes and bipartite coverings
Neighborly families of boxes and bipartite coverings Noga Alon Dedicated to Professor Paul Erdős on the occasion of his 80 th birthday Abstract A bipartite covering of order k of the complete graph K n
More informationDECOMPOSITIONS OF MULTIGRAPHS INTO PARTS WITH THE SAME SIZE
Discussiones Mathematicae Graph Theory 30 (2010 ) 335 347 DECOMPOSITIONS OF MULTIGRAPHS INTO PARTS WITH THE SAME SIZE Jaroslav Ivančo Institute of Mathematics P.J. Šafári University, Jesenná 5 SK-041 54
More informationRegular Honest Graphs, Isoperimetric Numbers, and Bisection of Weighted Graphs
Regular Honest Graphs, Isoperimetric Numbers, and Bisection of Weighted Graphs Noga Alon Department of Mathematics Raymond and Beverly Sackler Faculty of Exact Sciences Peter Hamburger Department of Mathematical
More informationProbabilistic Proofs of Existence of Rare Events. Noga Alon
Probabilistic Proofs of Existence of Rare Events Noga Alon Department of Mathematics Sackler Faculty of Exact Sciences Tel Aviv University Ramat-Aviv, Tel Aviv 69978 ISRAEL 1. The Local Lemma In a typical
More informationOn a list-coloring problem
On a list-coloring problem Sylvain Gravier Frédéric Maffray Bojan Mohar December 24, 2002 Abstract We study the function f(g) defined for a graph G as the smallest integer k such that the join of G with
More informationSTABLE KNESER HYPERGRAPHS AND IDEALS IN N WITH THE NIKODÝM PROPERTY
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 00, Number 0, Pages 000 000 S 0002-9939(XX)0000-0 STABLE KNESER HYPERGRAPHS AND IDEALS IN N WITH THE NIKODÝM PROPERTY NOGA ALON, LECH DREWNOWSKI,
More informationOn the Dynamic Chromatic Number of Graphs
On the Dynamic Chromatic Number of Graphs Maryam Ghanbari Joint Work with S. Akbari and S. Jahanbekam Sharif University of Technology m_phonix@math.sharif.ir 1. Introduction Let G be a graph. A vertex
More informationA note on balanced bipartitions
A note on balanced bipartitions Baogang Xu a,, Juan Yan a,b a School of Mathematics and Computer Science Nanjing Normal University, 1 Ninghai Road, Nanjing, 10097, China b College of Mathematics and System
More informationConstructive bounds for a Ramsey-type problem
Constructive bounds for a Ramsey-type problem Noga Alon Michael Krivelevich Abstract For every fixed integers r, s satisfying r < s there exists some ɛ = ɛ(r, s > 0 for which we construct explicitly an
More informationAn approximate version of Hadwiger s conjecture for claw-free graphs
An approximate version of Hadwiger s conjecture for claw-free graphs Maria Chudnovsky Columbia University, New York, NY 10027, USA and Alexandra Ovetsky Fradkin Princeton University, Princeton, NJ 08544,
More informationAvoider-Enforcer games played on edge disjoint hypergraphs
Avoider-Enforcer games played on edge disjoint hypergraphs Asaf Ferber Michael Krivelevich Alon Naor July 8, 2013 Abstract We analyze Avoider-Enforcer games played on edge disjoint hypergraphs, providing
More informationA short course on matching theory, ECNU Shanghai, July 2011.
A short course on matching theory, ECNU Shanghai, July 2011. Sergey Norin LECTURE 3 Tight cuts, bricks and braces. 3.1. Outline of Lecture Ear decomposition of bipartite graphs. Tight cut decomposition.
More informationCIRCULAR CHROMATIC NUMBER AND GRAPH MINORS. Xuding Zhu 1. INTRODUCTION
TAIWANESE JOURNAL OF MATHEMATICS Vol. 4, No. 4, pp. 643-660, December 2000 CIRCULAR CHROMATIC NUMBER AND GRAPH MINORS Xuding Zhu Abstract. This paper proves that for any integer n 4 and any rational number
More informationCodes and Xor graph products
Codes and Xor graph products Noga Alon Eyal Lubetzky November 0, 005 Abstract What is the maximum possible number, f 3 (n), of vectors of length n over {0, 1, } such that the Hamming distance between every
More informationOn the number of cycles in a graph with restricted cycle lengths
On the number of cycles in a graph with restricted cycle lengths Dániel Gerbner, Balázs Keszegh, Cory Palmer, Balázs Patkós arxiv:1610.03476v1 [math.co] 11 Oct 2016 October 12, 2016 Abstract Let L be a
More informationPaths and cycles in extended and decomposable digraphs
Paths and cycles in extended and decomposable digraphs Jørgen Bang-Jensen Gregory Gutin Department of Mathematics and Computer Science Odense University, Denmark Abstract We consider digraphs called extended
More informationApproximation algorithms for cycle packing problems
Approximation algorithms for cycle packing problems Michael Krivelevich Zeev Nutov Raphael Yuster Abstract The cycle packing number ν c (G) of a graph G is the maximum number of pairwise edgedisjoint cycles
More informationLongest paths in strong spanning oriented subgraphs of strong semicomplete multipartite digraphs
Longest paths in strong spanning oriented subgraphs of strong semicomplete multipartite digraphs Gregory Gutin Department of Mathematical Sciences Brunel, The University of West London Uxbridge, Middlesex,
More informationChromatic Ramsey number of acyclic hypergraphs
Chromatic Ramsey number of acyclic hypergraphs András Gyárfás Alfréd Rényi Institute of Mathematics Hungarian Academy of Sciences Budapest, P.O. Box 127 Budapest, Hungary, H-1364 gyarfas@renyi.hu Alexander
More informationProblems and results in Extremal Combinatorics, Part I
Problems and results in Extremal Combinatorics, Part I Noga Alon Abstract Extremal Combinatorics is an area in Discrete Mathematics that has developed spectacularly during the last decades. This paper
More informationOn the Maximum Number of Hamiltonian Paths in Tournaments
On the Maximum Number of Hamiltonian Paths in Tournaments Ilan Adler Noga Alon Sheldon M. Ross August 2000 Abstract By using the probabilistic method, we show that the maximum number of directed Hamiltonian
More informationRamsey-nice families of graphs
Ramsey-nice families of graphs Ron Aharoni Noga Alon Michal Amir Penny Haxell Dan Hefetz Zilin Jiang Gal Kronenberg Alon Naor August 22, 2017 Abstract For a finite family F of fixed graphs let R k (F)
More informationOn (δ, χ)-bounded families of graphs
On (δ, χ)-bounded families of graphs András Gyárfás Computer and Automation Research Institute Hungarian Academy of Sciences Budapest, P.O. Box 63 Budapest, Hungary, H-1518 gyarfas@sztaki.hu Manouchehr
More informationList of Theorems. Mat 416, Introduction to Graph Theory. Theorem 1 The numbers R(p, q) exist and for p, q 2,
List of Theorems Mat 416, Introduction to Graph Theory 1. Ramsey s Theorem for graphs 8.3.11. Theorem 1 The numbers R(p, q) exist and for p, q 2, R(p, q) R(p 1, q) + R(p, q 1). If both summands on the
More informationApproximation Algorithms for Asymmetric TSP by Decomposing Directed Regular Multigraphs
Approximation Algorithms for Asymmetric TSP by Decomposing Directed Regular Multigraphs Haim Kaplan Tel-Aviv University, Israel haimk@post.tau.ac.il Nira Shafrir Tel-Aviv University, Israel shafrirn@post.tau.ac.il
More informationGraphs with large maximum degree containing no odd cycles of a given length
Graphs with large maximum degree containing no odd cycles of a given length Paul Balister Béla Bollobás Oliver Riordan Richard H. Schelp October 7, 2002 Abstract Let us write f(n, ; C 2k+1 ) for the maximal
More informationRao s degree sequence conjecture
Rao s degree sequence conjecture Maria Chudnovsky 1 Columbia University, New York, NY 10027 Paul Seymour 2 Princeton University, Princeton, NJ 08544 July 31, 2009; revised December 10, 2013 1 Supported
More informationOn the Maximum Number of Hamiltonian Paths in Tournaments
On the Maximum Number of Hamiltonian Paths in Tournaments Ilan Adler, 1 Noga Alon,, * Sheldon M. Ross 3 1 Department of Industrial Engineering and Operations Research, University of California, Berkeley,
More informationThe Reduction of Graph Families Closed under Contraction
The Reduction of Graph Families Closed under Contraction Paul A. Catlin, Department of Mathematics Wayne State University, Detroit MI 48202 November 24, 2004 Abstract Let S be a family of graphs. Suppose
More informationOn saturation games. May 11, Abstract
On saturation games Dan Hefetz Michael Krivelevich Alon Naor Miloš Stojaković May 11, 015 Abstract A graph G = (V, E) is said to be saturated with respect to a monotone increasing graph property P, if
More informationCographs; chordal graphs and tree decompositions
Cographs; chordal graphs and tree decompositions Zdeněk Dvořák September 14, 2015 Let us now proceed with some more interesting graph classes closed on induced subgraphs. 1 Cographs The class of cographs
More informationAll Ramsey numbers for brooms in graphs
All Ramsey numbers for brooms in graphs Pei Yu Department of Mathematics Tongji University Shanghai, China yupeizjy@16.com Yusheng Li Department of Mathematics Tongji University Shanghai, China li yusheng@tongji.edu.cn
More informationCoins with arbitrary weights. Abstract. Given a set of m coins out of a collection of coins of k unknown distinct weights, we wish to
Coins with arbitrary weights Noga Alon Dmitry N. Kozlov y Abstract Given a set of m coins out of a collection of coins of k unknown distinct weights, we wish to decide if all the m given coins have the
More informationPacking Ferrers Shapes
Packing Ferrers Shapes Noga Alon Miklós Bóna Joel Spencer February 22, 2002 Abstract Answering a question of Wilf, we show that if n is sufficiently large, then one cannot cover an n p(n) rectangle using
More informationCompatible Hamilton cycles in Dirac graphs
Compatible Hamilton cycles in Dirac graphs Michael Krivelevich Choongbum Lee Benny Sudakov Abstract A graph is Hamiltonian if it contains a cycle passing through every vertex exactly once. A celebrated
More informationOut-colourings of Digraphs
Out-colourings of Digraphs N. Alon J. Bang-Jensen S. Bessy July 13, 2017 Abstract We study vertex colourings of digraphs so that no out-neighbourhood is monochromatic and call such a colouring an out-colouring.
More informationGraphs & Algorithms: Advanced Topics Nowhere-Zero Flows
Graphs & Algorithms: Advanced Topics Nowhere-Zero Flows Uli Wagner ETH Zürich Flows Definition Let G = (V, E) be a multigraph (allow loops and parallel edges). An (integer-valued) flow on G (also called
More informationParity Versions of 2-Connectedness
Parity Versions of 2-Connectedness C. Little Institute of Fundamental Sciences Massey University Palmerston North, New Zealand c.little@massey.ac.nz A. Vince Department of Mathematics University of Florida
More informationProperly Colored Subgraphs and Rainbow Subgraphs in Edge-Colorings with Local Constraints
Properly Colored Subgraphs and Rainbow Subgraphs in Edge-Colorings with Local Constraints Noga Alon, 1, * Tao Jiang, 2, Zevi Miller, 2 Dan Pritikin 2 1 Department of Mathematics, Sackler Faculty of Exact
More informationThe last fraction of a fractional conjecture
Author manuscript, published in "SIAM Journal on Discrete Mathematics 24, 2 (200) 699--707" DOI : 0.37/090779097 The last fraction of a fractional conjecture František Kardoš Daniel Král Jean-Sébastien
More informationLarge induced forests in sparse graphs
Large induced forests in sparse graphs Noga Alon, Dhruv Mubayi, Robin Thomas Published in J. Graph Theory 38 (2001), 113 123. Abstract For a graph G, leta(g) denote the maximum size of a subset of vertices
More informationApproximating the independence number via the ϑ-function
Approximating the independence number via the ϑ-function Noga Alon Nabil Kahale Abstract We describe an approximation algorithm for the independence number of a graph. If a graph on n vertices has an independence
More information4 CONNECTED PROJECTIVE-PLANAR GRAPHS ARE HAMILTONIAN. Robin Thomas* Xingxing Yu**
4 CONNECTED PROJECTIVE-PLANAR GRAPHS ARE HAMILTONIAN Robin Thomas* Xingxing Yu** School of Mathematics Georgia Institute of Technology Atlanta, Georgia 30332, USA May 1991, revised 23 October 1993. Published
More informationOn Some Three-Color Ramsey Numbers for Paths
On Some Three-Color Ramsey Numbers for Paths Janusz Dybizbański, Tomasz Dzido Institute of Informatics, University of Gdańsk Wita Stwosza 57, 80-952 Gdańsk, Poland {jdybiz,tdz}@inf.ug.edu.pl and Stanis
More informationAdvanced Topics in Discrete Math: Graph Theory Fall 2010
21-801 Advanced Topics in Discrete Math: Graph Theory Fall 2010 Prof. Andrzej Dudek notes by Brendan Sullivan October 18, 2010 Contents 0 Introduction 1 1 Matchings 1 1.1 Matchings in Bipartite Graphs...................................
More informationMATH 2200 Final Review
MATH 00 Final Review Thomas Goller December 7, 01 1 Exam Format The final exam will consist of 8-10 proofs It will take place on Tuesday, December 11, from 10:30 AM - 1:30 PM, in the usual room Topics
More informationAdditive Latin Transversals
Additive Latin Transversals Noga Alon Abstract We prove that for every odd prime p, every k p and every two subsets A = {a 1,..., a k } and B = {b 1,..., b k } of cardinality k each of Z p, there is a
More information4 Packing T-joins and T-cuts
4 Packing T-joins and T-cuts Introduction Graft: A graft consists of a connected graph G = (V, E) with a distinguished subset T V where T is even. T-cut: A T -cut of G is an edge-cut C which separates
More informationConnectivity of addable graph classes
Connectivity of addable graph classes Paul Balister Béla Bollobás Stefanie Gerke July 6, 008 A non-empty class A of labelled graphs is weakly addable if for each graph G A and any two distinct components
More informationClimbing an Infinite Ladder
Section 5.1 Section Summary Mathematical Induction Examples of Proof by Mathematical Induction Mistaken Proofs by Mathematical Induction Guidelines for Proofs by Mathematical Induction Climbing an Infinite
More informationPartial cubes: structures, characterizations, and constructions
Partial cubes: structures, characterizations, and constructions Sergei Ovchinnikov San Francisco State University, Mathematics Department, 1600 Holloway Ave., San Francisco, CA 94132 Abstract Partial cubes
More informationINDUCED CYCLES AND CHROMATIC NUMBER
INDUCED CYCLES AND CHROMATIC NUMBER A.D. SCOTT DEPARTMENT OF MATHEMATICS UNIVERSITY COLLEGE, GOWER STREET, LONDON WC1E 6BT Abstract. We prove that, for any pair of integers k, l 1, there exists an integer
More informationMinimal Paths and Cycles in Set Systems
Minimal Paths and Cycles in Set Systems Dhruv Mubayi Jacques Verstraëte July 9, 006 Abstract A minimal k-cycle is a family of sets A 0,..., A k 1 for which A i A j if and only if i = j or i and j are consecutive
More informationEquitable coloring of random graphs
Michael Krivelevich 1, Balázs Patkós 2 1 Tel Aviv University, Tel Aviv, Israel 2 Central European University, Budapest, Hungary Phenomena in High Dimensions, Samos 2007 A set of vertices U V (G) is said
More informationProper connection number and 2-proper connection number of a graph
Proper connection number and 2-proper connection number of a graph arxiv:1507.01426v2 [math.co] 10 Jul 2015 Fei Huang, Xueliang Li, Shujing Wang Center for Combinatorics and LPMC-TJKLC Nankai University,
More informationDecomposing dense bipartite graphs into 4-cycles
Decomposing dense bipartite graphs into 4-cycles Nicholas J. Cavenagh Department of Mathematics The University of Waikato Private Bag 3105 Hamilton 3240, New Zealand nickc@waikato.ac.nz Submitted: Jun
More informationEven Pairs and Prism Corners in Square-Free Berge Graphs
Even Pairs and Prism Corners in Square-Free Berge Graphs Maria Chudnovsky Princeton University, Princeton, NJ 08544 Frédéric Maffray CNRS, Laboratoire G-SCOP, University of Grenoble-Alpes, France Paul
More informationThe number of edge colorings with no monochromatic cliques
The number of edge colorings with no monochromatic cliques Noga Alon József Balogh Peter Keevash Benny Sudaov Abstract Let F n, r, ) denote the maximum possible number of distinct edge-colorings of a simple
More informationThree-coloring triangle-free graphs on surfaces V. Coloring planar graphs with distant anomalies
Three-coloring triangle-free graphs on surfaces V. Coloring planar graphs with distant anomalies Zdeněk Dvořák Daniel Král Robin Thomas Abstract We settle a problem of Havel by showing that there exists
More informationON THE CORE OF A GRAPHf
ON THE CORE OF A GRAPHf By FRANK HARARY and MICHAEL D. PLUMMER [Received 8 October 1965] 1. Introduction Let G be a graph. A set of points M is said to cover all the lines of G if every line of G has at
More informationSupereulerian planar graphs
Supereulerian planar graphs Hong-Jian Lai and Mingquan Zhan Department of Mathematics West Virginia University, Morgantown, WV 26506, USA Deying Li and Jingzhong Mao Department of Mathematics Central China
More informationarxiv: v1 [math.co] 22 Jan 2018
arxiv:1801.07025v1 [math.co] 22 Jan 2018 Spanning trees without adjacent vertices of degree 2 Kasper Szabo Lyngsie, Martin Merker Abstract Albertson, Berman, Hutchinson, and Thomassen showed in 1990 that
More informationNowhere 0 mod p dominating sets in multigraphs
Nowhere 0 mod p dominating sets in multigraphs Raphael Yuster Department of Mathematics University of Haifa at Oranim Tivon 36006, Israel. e-mail: raphy@research.haifa.ac.il Abstract Let G be a graph with
More informationDecomposing oriented graphs into transitive tournaments
Decomposing oriented graphs into transitive tournaments Raphael Yuster Department of Mathematics University of Haifa Haifa 39105, Israel Abstract For an oriented graph G with n vertices, let f(g) denote
More informationDisjoint Hamiltonian Cycles in Bipartite Graphs
Disjoint Hamiltonian Cycles in Bipartite Graphs Michael Ferrara 1, Ronald Gould 1, Gerard Tansey 1 Thor Whalen Abstract Let G = (X, Y ) be a bipartite graph and define σ (G) = min{d(x) + d(y) : xy / E(G),
More informationZero-one laws for multigraphs
Zero-one laws for multigraphs Caroline Terry University of Illinois at Chicago AMS Central Fall Sectional Meeting 2015 Caroline Terry (University of Illinois at Chicago) Zero-one laws for multigraphs October
More informationDiscrete Mathematics. The average degree of a multigraph critical with respect to edge or total choosability
Discrete Mathematics 310 (010 1167 1171 Contents lists available at ScienceDirect Discrete Mathematics journal homepage: www.elsevier.com/locate/disc The average degree of a multigraph critical with respect
More informationForcing unbalanced complete bipartite minors
Forcing unbalanced complete bipartite minors Daniela Kühn Deryk Osthus Abstract Myers conjectured that for every integer s there exists a positive constant C such that for all integers t every graph of
More informationOn the mean connected induced subgraph order of cographs
AUSTRALASIAN JOURNAL OF COMBINATORICS Volume 71(1) (018), Pages 161 183 On the mean connected induced subgraph order of cographs Matthew E Kroeker Lucas Mol Ortrud R Oellermann University of Winnipeg Winnipeg,
More informationRelating minimum degree and the existence of a k-factor
Relating minimum degree and the existence of a k-factor Stephen G Hartke, Ryan Martin, and Tyler Seacrest October 6, 010 Abstract A k-factor in a graph G is a spanning regular subgraph in which every vertex
More informationGraph homomorphism into an odd cycle
Graph homomorphism into an odd cycle Hong-Jian Lai West Virginia University, Morgantown, WV 26506 EMAIL: hjlai@math.wvu.edu Bolian Liu South China Normal University, Guangzhou, P. R. China EMAIL: liubl@hsut.scun.edu.cn
More informationEfficient testing of large graphs
Efficient testing of large graphs Noga Alon Eldar Fischer Michael Krivelevich Mario Szegedy Abstract Let P be a property of graphs. An ɛ-test for P is a randomized algorithm which, given the ability to
More informationA necessary and sufficient condition for the existence of a spanning tree with specified vertices having large degrees
A necessary and sufficient condition for the existence of a spanning tree with specified vertices having large degrees Yoshimi Egawa Department of Mathematical Information Science, Tokyo University of
More information