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In this paper, we give evidence for the problems Disjoint Cycles and Disjoint Paths that they cannot be preprocessed in polynomial time such that resulting instances always have a size bounded by a polynomial in a specified parameter (or, in short: do not have a polynomial kernel); these results are assuming the validity of certain complexity theoretic… (More)

We prove that given an undirected graph <i>G</i> on <i>n</i> vertices and an integer <i>k</i>, one can compute, in polynomial time in <i>n</i>, a graph <i>G′</i> with at most 4<i>k</i><sup>2</sup> vertices and an integer <i>k′</i> such that <i>G</i> has a feedback vertex set of size at most <i>k</i> iff <i>G′</i> has a feedback vertex set… (More)

We prove that every graph G has a vertex partition into a cycle and an anticycle (a cycle in the complement of G). Emptyset, singletons and edges are considered as cycles. This problem was posed by Lehel and shown to be true for very large graphs by Luczak, Rödl and Szemerédi [7], and more recently for large graphs by Allen [1]. Many questions deal with the… (More)

We consider the problem of finding a large or dense triangle-free subgraph in a given graph G. In response to a question of P. Erd˝ os, we prove that, if the minimum degree of G is at least 17|V (G)|/20, the largest triangle-free subgraphs are precisely the largest bipartite subgraphs in G. We investigate in particular the case where G is a complete… (More)

We prove that given an undirected graph G on n vertices and an integer k, one can compute in polynomial time in n a graph G with at most 5k 2 +k vertices and an integer k such that G has a feedback vertex set of size at most k iff G has a feedback vertex set of size at most k. This result improves a previous O(k 11) kernel of Burrage et al. [6], and a more… (More)

We prove that with three exceptions, every tournament of order n contains each oriented path of order n. The exceptions are the antidirected paths in the 3-cycle, in the regular tournament on 5 vertices, and in the Paley tournament on 7 vertices. Tournaments are very rich structures and many questions deal with their subgraphs. In particular, much work has… (More)

We give a short constructive proof of a theorem of Fisher: every tournament contains a vertex whose second outneighborhood is as large as its ®rst outneighborhood. Moreover, we exhibit two such vertices provided that the tournament has no dominated vertex. The proof makes use of median orders. A second application of median orders is that every tournament… (More)

Let G=(V,E) be a graph on n vertices and R be a set of pairs of vertices in V called requests. A multicut is a subset F of E such that every request xy of R is cut by F, i.e. every xy-path of G intersects F. We show that there exists an O(f(k)n<sup>c</sup>) algorithm which decides if there exists a multicut of size at most k. In other words, the Multicut… (More)

Adapting the method introduced in Graph Minors X, we propose a new proof of the duality between the bramble-number of a graph and its tree-width. Our approach is based on a new definition of submodularity on partition functions which naturally extends the usual one on set functions. The proof does not rely on Menger's theorem, and thus greatly generalises… (More)

Answering a question of Kalai and Meshulam, we prove that graphs without induced cycles of length 3k have bounded chromatic number. This implies the very first case of a much broader question asserting that every graph with large chromatic number induces a graph H such that the sum of the Betti numbers of the independence complex of H is also large.