Seminars and Colloquia by Series

Thursday, March 6, 2014 - 12:05 , Location: Skiles 005 , Spencer Backman , Math, GT , Organizer: Robin Thomas
This talk is a sequel to the speaker's previous lecture given in the January 31st Combinatorics Seminar, but attendance at the first talk is not assumed. We begin by carefully reviewing our generalized cycle-cocyle reversal system for partial graph orientations. A self contained description of Baker and Norin's Riemann-Roch formula for graphs is given using their original chip-firing language. We then explain how to reinterpret and reprove this theorem using partial graph orientations. In passing, the Baker-Norin rank of a partial orientation is shown to be one less than the minimum number of directed paths which need to be reversed in the generalized cycle-cocycle reversal system to produce an acyclic partial orientation. We conclude with an overview of how these results extend to the continuous setting of metric graphs (abstract tropical curves).
Thursday, February 27, 2014 - 12:05 , Location: Skiles 005 , Paul Wollan , University of Rome "La Sapienza" , Organizer: Robin Thomas
Consider a graph G and a specified subset A of vertices. An A-path is a path with both ends in A and no internal vertex in A. Gallai showed that there exists a min-max formula for the maximum number of pairwise disjoint A-paths. More recent work has extended this result, considering disjoint A-paths which satisfy various additional properties. We consider the following model. We are given a list of {(s_i, t_i): 0< i < k} of pairs of vertices in A, consider the question of whether there exist many pairwise disjoint A-paths P_1,..., P_t such that for all j, the ends of P_j are equal to s_i and t_i for some value i. This generalizes the disjoint paths problem and is NP-hard if k is not fixed. Thus, we cannot hope for an exact min-max theorem. We further restrict the question, and ask if there either exist t pairwise disjoint such A-paths or alternatively, a bounded set of f(t) vertices intersecting all such paths. In general, there exist examples where no such function f(t) exists; we present an exact characterization of when such a function exists. This is joint work with Daniel Marx.
Wednesday, November 13, 2013 - 16:05 , Location: Skiles 005 , Hao Huang , Institute for Advanced Study and DIMACS , Organizer: Robin Thomas
An r-unform n-vertex hypergraph H is said to have the Manickam-Miklos-Singhi (MMS) property if for every assignment of weights to its vertices with nonnegative sum, the number of edges whose total weight is nonnegative is at least the minimum degree of H. In this talk I will show that for n>10r^3, every r-uniform n-vertex hypergraph with equal codegrees has the MMS property, and the bound on n is essentially tight up to a constant factor. An immediate corollary of this result is the vector space Manickam-Miklos-Singhi conjecture which states that for n>=4k and any weighting on the 1-dimensional subspaces of F_q^n with nonnegative sum, the number of nonnegative k-dimensional subspaces is at least ${n-1 \brack k-1}_q$. I will also discuss two additional generalizations, which can be regarded as analogues of the Erdos-Ko-Rado theorem on k-intersecting families. This is joint work with Benny Sudakov. 
Tuesday, September 24, 2013 - 12:05 , Location: Skiles 005 , Zdenek Dvorak , Charles University , Organizer: Robin Thomas
By the 4-color theorem, every planar graph on n vertices has an independent set of size at least n/4. Finding a simple proof of this fact is a long-standing open problem. Furthermore, no polynomial-time algorithm to decide whether a planar graph has an independent set of size at least (n+1)/4 is known. We study the analogous problem for triangle-free planar graphs. By Grotzsch' theorem, each such graph on n vertices has an independent set of size at least n/3, and this can be easily improved to a tight bound of (n+1)/3. We show that for every k, a triangle-free planar graph of sufficiently large tree-width has an independent set of size at least (n+k)/3, thus giving a polynomial-time algorithm to decide the existence of such a set. Furthermore, we show that there exists a constant c < 3 such that every planar graph of girth at least five has an independent set of size at least n/c.Joint work with Matthias Mnich.
Thursday, September 19, 2013 - 12:05 , Location: Skiles 005 , Paul Wollan , School of Mathematics, Georgia Tech and University of Rome, Italy , Organizer: Robin Thomas
While Robertson and Seymour showed that graphs are well-quasi-ordered under the minor relation, it is well known that directed graphs are not. We will present an exact characterization of the minor-closed sets of directed graphs which are well-quasi-ordered. This is joint work with M. Chudnovsky, S. Oum, I. Muzi, and P. Seymour.
Tuesday, September 10, 2013 - 12:05 , Location: Skiles 005 , Martin Loebl , Charles University , Organizer: Robin Thomas
We prove the dcdc conjecture in a class of lean fork graphs, argue that this class is substantial and show a path towards the complete solution. Joint work with Andrea Jimenez.
Thursday, April 18, 2013 - 12:05 , Location: Skiles 005 , Daniel Kral , University of Warwick , Organizer: Robin Thomas
A systematic study of large combinatorial objects has recently led to discovering many connections between discrete mathematics and analysis. In this talk, we apply analytic methods to permutations. In particular, we associate every sequence of permutations with a measure on a unit square and show the following: if the density of every 4-element subpermutation in a permutation p is 1/4!+o(1), then the density of every k-element subpermutation is 1/k!+o(1). This answers a question of Graham whether quasirandomness of a permutation is captured by densities of its 4-element subpermutations. The result is based on a joint work with Oleg Pikhurko.
Thursday, April 4, 2013 - 12:05 , Location: Skiles 005 , Dhruv Mubayi , University of Illinois at Chicago , Organizer: Robin Thomas
Since the foundational results of Thomason and Chung-Graham-Wilson on quasirandom graphs over 20 years ago, there has been a lot of effort by many researchers to extend the theory to hypergraphs. I will present some of this history, and then describe our recent results that provide such a generalization and unify much of the previous work. One key new aspect in the theory is a systematic study of hypergraph eigenvalues first introduced by Friedman and Wigderson. This is joint work with John Lenz.
Thursday, March 28, 2013 - 12:05 , Location: Skiles 005 , Peter Whalen , Georgia Tech , Organizer: Robin Thomas
 We show that any internally 4-connected non-planar bipartite graph contains a subdivision of K3,3 in which each subdivided path contains an even number of vertices. In addition to being natural, this result has broader applications in matching theory: for example, finding such a subdivision of K3,3 is the first step in an algorithm for determining whether or not a bipartite graph is Pfaffian. This is joint work with Robin Thomas. 
Tuesday, March 26, 2013 - 12:05 , Location: Skiles 005 , Bernard Lidicky , University of Illinois at Urbana-Champaign , Organizer: Robin Thomas
A recent lower bound on the number of edges in a k-critical n-vertex graph by Kostochka and Yancey yields a half-page proof of the celebrated Grotzsch Theorem that every planar triangle-free graph is 3-colorable. We use the same bound to give short proofs of other known theorems on 3-coloring of planar graphs, among whose is the Grunbaum-Aksenov Theorem that every planar with at most three triangles is 3-colorable. We also prove the new result that every graph obtained from a triangle-free planar graph by adding a vertex of degree at most four is 3-colorable. Joint work with O. Borodin, A. Kostochka and M. Yancey.