## Seminars and Colloquia by Series

Thursday, November 8, 2012 - 12:05 , Location: Skiles 005 , Hein van der Holst , Georgia State University , Organizer: Robin Thomas
A signed graph is a pair $(G,\Sigma)$ where $G$ is an undirected graph (in which
parallel edges are permitted, but loops are not) and $\Sigma \subseteq E(G)$.
The edges in $\Sigma$ are called odd and the other edges are called even. A
cycle of $G$ is called odd if it has an odd number of odd edges. If $U\subseteq V(G)$, then re-signing $(G,\Sigma)$ on $U$ gives the signed graph
$(G,\Sigma\Delta \delta(U))$. A signed graph is a minor of $(G,\Sigma)$ if it
comes from $(G,\Sigma)$ by a series of re-signing, deletions of edges and
isolated vertices, and contractions of even edges.

If $(G,\Sigma)$ is a signed graph with $n$ vertices, $S(G,\Sigma)$ is the set of
all symmetric $n\times n$ matrices $A=[a_{i,j}]$ with $a_{i,j} > 0$ if $i$ and
$j$ are connected by only odd edges, $a_{i,j} < 0$ if $i$ and $j$ are connected
by only even edges, $a_{i,j}\in \mathbb{R}$ if $i$ and $j$ are connected by
both even and odd edges, $a_{i,j}=0$ if $i$ and $j$ are not connected by any
edges, and $a_{i,i} \in \mathbb{R}$ for all vertices $i$.

The stable inertia set, $I_s(G,\Sigma)$, of a signed graph $(G,\Sigma)$ is the
set of all pairs $(p,q)$ such that there exists a matrix $A\in S(G,\Sigma)$
that has the Strong Arnold Hypothesis, and $p$ positive and $q$ negative
eigenvalues. The stable inertia set of a signed graph forms a generalization of
$\mu(G)$, $\nu(G)$ (introduced by Colin de Verdi\ere), and $\xi(G)$
(introduced by Barioli, Fallat, and Hogben).

A specialization of $I_s(G,\Sigma)$ is $\nu(G,\Sigma)$, which is defined as the
maximum of the nullities of positive definite matrices $A\in S(G,\Sigma)$ that
have the Strong Arnold Hypothesis.
This invariant is closed under taking minors, and characterizes signed graphs
with no odd cycles as those signed graphs $(G,\Sigma)$ with $\nu(G,\Sigma)\leq 1$, and signed graphs with no odd-$K_4$- and no odd-$K^2_3$-minor as those
signed graphs $(G,\Sigma)$ with $\nu(G,\Sigma)\leq 2$. In this talk we will
discuss $I_s(G,\Sigma)$, $\nu(G,\Sigma)$ and these characterizations.

Joint work with Marina Arav, Frank Hall, and Zhongshan Li.

Thursday, October 25, 2012 - 12:05 , Location: Skiles 005 , William T. Trotter , Math, GT , Organizer: Robin Thomas
Over the past 40 years, researchers
have made many connections between the
dimension of posets and the issue of planarity
for graphs and diagrams, but there appears
to be little work connecting
dimension to structural graph theory. This situation
has changed dramatically in the last several months. At the Robin Thomas birthday conference, Gwenael
Joret, made the following striking conjecture, which
has now been turned into a theorem: The dimension
of a poset is bounded in terms of its height and the
tree-width of its cover graph. In this talk, I will present the proof of this result. The general contours of
the argument should be accessible to graph theorists and combinatorists (faculty and students) without deep knowledge of either dimension or tree-width.

The proof of the theorem was
accomplished by a team of six researchers: Gwenael Joret, Piotr Micek, Kevin Milans, Tom Trotter,
Bartosz Walczak and Ruidong Wang.

Thursday, April 26, 2012 - 13:05 , Location: Skiles 005 , Peter Whalen , Math, GT , Organizer: Robin Thomas
We show that any n-vertex complete graph with edges colored with three
colors contains a set of at most four vertices such that the number of the
neighbors of these vertices in one of the colors is at least 2n/3. The
previous best value proved by Erdos et al in 1989 is 22. It is conjectured
that three vertices suffice. This is joint work with Daniel Kral,
Chun-Hung Liu, Jean-Sebastien Sereni, and Zelealem Yilma.
Thursday, April 19, 2012 - 12:05 , Location: Skiles 005 , Paul Wollan , ISyE, GT and The Sapienza University of Rome , Organizer: Robin Thomas
A graph $G$ contains a graph $H$ as an immersion if there exist distinct vertices $\pi(v) \in V(G)$ for every vertex $v \in V(H)$ and paths $P(e)$ in $G$ for every $e \in E(H)$ such that the path $P(uv)$ connects the vertices $\pi(u)$ and $\pi(v)$ in $G$ and furthermore the paths $\{P(e):e \in E(H)\}$ are pairwise edge disjoint. Thus, graph immersion can be thought of as a generalization of subdivision containment where the paths linking the pairs of branch vertices are required to be pairwise edge disjoint instead of pairwise internally vertex disjoint. We will present a simple structure theorem for graphs excluding a fixed $K_t$ as an immersion. The structure theorem gives rise to a model of tree-decompositions based on edge cuts instead of vertex cuts. We call these decompositions tree-cut decompositions, and give an appropriate definition for the width of such a decomposition. We will present a `grid" theorem for graph immersions with respect to the tree-cut width.

This is joint work with Paul Seymour.

Thursday, March 29, 2012 - 12:05 , Location: Skiles 005 , Laszlo Vegh , College of Computing, Georgia Tech , Organizer: Robin Thomas
In the node-connectivity augmentation problem, we want to add a minimum
number of new edges to an undirected graph to make it k-node-connected.
The complexity of this question is still open, although the analogous questions
of both directed and undirected edge-connectivity and directed
node-connectivity augmentation are known to be polynomially solvable.

I present a min-max formula and a polynomial time algorithm for the
special case when the input graph is already (k-1)-connected. The formula has
been conjectured by Frank and Jordan in 1994.

In the first lecture, I presented previous results on the other connectivity augmentation variants.
In the second part, I shall present my min-max formula and the main ideas of the proof.

Thursday, March 8, 2012 - 12:05 , Location: Skiles 005 , Laszlo Vegh , CoC, GT , Organizer: Robin Thomas
In the node-connectivity augmentation problem, we want to add a minimum
number of new edges to an undirected graph to make it k-node-connected.
The complexity of this question is still open, although the analogous questions
of both directed and undirected edge-connectivity and directed
node-connectivity augmentation are known to be polynomially solvable.

I present a min-max formula and a polynomial time algorithm for the
special case when the input graph is already (k-1)-connected. The formula has
been conjectured by Frank and Jordan in 1994.

In the first lecture, I shall investigate the background, present some results on the previously
solved connectivity augmentation cases, and exhibit examples motivating the complicated
min-max formula of my paper.

Thursday, February 23, 2012 - 12:05 , Location: Skiles 005 , Hanno Lefmann , Chemnitz University of Technology, Germany , Organizer: Prasad Tetali
Thursday, February 16, 2012 - 12:05 , Location: Skiles 006 , Arkadiusz Pawlik , Jagiellonian University, Krakow, Poland , Organizer: Robin Thomas
We consider intersection graphs of
families of straight line segments in the euclidean
plane and show that for every integer k, there is a
family S of line segments so that the intersection graph
G of the family S is triangle-free and has chromatic
number at least k. This result settles a conjecture
of Erdos and has a number of applications to
other classes of intersection graphs.
Thursday, November 17, 2011 - 12:05 , Location: Skiles 005 , Iain Moffatt , University of South Alabama , Organizer: Robin Thomas
A classical result in graph theory states that, if G is a plane graph,
then G is Eulerian if and only if its dual, G*, is bipartite. I will
talk about an extension of this well-known result to partial duality.
(Where, loosely speaking, a partial dual of an embedded graph G is a graph
obtained by forming the dual with respect to only a subset of edges of G.)
I will extend the above classical connection between bipartite and Eulerian
plane graphs, by providing a necessary and sufficient condition for the
partial dual of a plane graph to be Eulerian or bipartite. I will then go on
to describe how the bipartite partial duals of a plane graph G are
completely characterized by circuits in its medial graph G_m.

This is joint work with Stephen Huggett.

Tuesday, October 25, 2011 - 12:05 , Location: Skiles 006 , Andrew King , Simon Fraser University , Organizer: Robin Thomas
Chudnovsky and Seymour's structure theorem for quasi-line graphs has led to
a multitude of recent results that exploit two structural operations: compositions of strips and thickenings. In this paper we prove that
compositions of linear interval strips have a unique optimal strip
decomposition in the absence of a specific degeneracy, and that every
claw-free graph has a unique optimal antithickening, where our two
definitions of optimal are chosen carefully to respect the structural
foundation of the graph. Furthermore, we give algorithms to find the optimal
strip decomposition in O(nm) time and find the optimal antithickening in
O(m2) time. For the sake of both completeness and ease of proof, we
prove stronger results in the more general setting of trigraphs. This gives
a comprehensive "black box" for decomposing quasi-line graphs that is not
only useful for future work but also improves the complexity of some
previous algorithmic results.

Joint work with Maria Chudnovsky.