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Series: Combinatorics Seminar

I will survey the major results in graph and hypergraph Ramsey theory and present some recent results on hypergraph Ramsey numbers. This includes a hypergraph generalization of the graph Ramsey number R(3,t) proved recently with Kostochka and Verstraete. If time permits some proofs will be presented.

Series: Combinatorics Seminar

Series: Combinatorics Seminar

Let A be a multiplicative subgroup of Z_p^*. Define the k-fold
sumset of A to be kA={x_1+...+x_k:x_1,...,x_k in A}. Recently, Shkredov
has shown that |2A| >> |A|^(8/5-\epsilon) for |A| < p^(9/17). In this talk we
will discuss extending this result to hold for |A| < p^(5/9). In addition,
we will show that 6A contains Z_p^* for |A| > p^(33/71 +\epsilon).

Series: Combinatorics Seminar

For two graphs, G and F, we write G\longrightarrow F if every
2-coloring of the edges of G results in a monochromatic copy of F.
A graph G is k-Folkman if G\longrightarrow K_k and G\not\supset K_{k+1}. We
show that there is a constant c > 0 such that for every k \ge 2 there exists a
k-Folkman graph on at most 2^{k^{ck^2}} vertices. Our probabilistic proof is
based on a careful analysis of the growth of constants in a modified proof of the
result by Rodl and the speaker from 1995 establishing a threshold for the Ramsey
property of a binomial random graph G(n,p).
Thus, at the same time, we provide a new proof of that result (for two colors) which
avoids the use of regularity lemma.
This is joint work with Vojta Rodl and Mathias Schacht.

Series: Combinatorics Seminar

For a given finite graph G of minimum degree at least k, let
G_{p} be a random subgraph of G obtained by taking each edge
independently with probability p. We prove that (i) if p \ge
\omega/k for a function \omega=\omega(k) that tends to infinity
as k does, then G_p asymptotically almost surely contains a
cycle (and thus a path) of length at least (1-o(1))k, and (ii) if
p \ge (1+o(1))\ln k/k, then G_p asymptotically almost surely
contains a path of length at least k. Our theorems extend
classical results on paths and cycles in the binomial random graph,
obtained by taking G to be the complete graph on k+1 vertices.
Joint w/ Michael Krivelevich (Tel Aviv), Benny Sudakov (UCLA).

Series: Combinatorics Seminar

We consider higher order Markov random fields to study independent sets in
regular graphs of large girth (i.e. Bethe lattice, Cayley tree). We give
sufficient conditions for a second-order homogenous isotropic Markov
random field to exhibit long-range boundary independence (i.e. decay of
correlations, unique infinite-volume Gibbs measure), and give both
necessary and sufficient conditions when the relevant clique potentials of
the corresponding Gibbs measure satisfy a log-convexity assumption. We
gain further insight into this characterization by interpreting our model
as a multi-dimensional perturbation of the hardcore model, and (under a
convexity assumption) give a simple polyhedral characterization for those
perturbations (around the well-studied critical activity of the hardcore
model) which maintain long-range boundary independence. After identifying
several features of this polyhedron, we also characterize (again as a
polyhedral set) how one can change the occupancy probabilities through
such a perturbation. We then use linear programming to analyze the
properties of this set of attainable probabilities, showing that although
one cannot acheive denser independent sets, it is possible to optimize the
number of excluded nodes which are adjacent to no included nodes.

Series: Combinatorics Seminar

A permutation of the set {1,2,...,n} is connected if there is no k < n such
that the set of the first k numbers is invariant as a set under the
permutation. For each permutation, there is a corresponding graph whose
vertices are the letters of the permutation and whose edges correspond to
the inversions in the permutation. In this way, connected permutations
correspond to connected permutation graphs.
We find a growth process of a random permutation in which we start with the
identity permutation on a fixed set of letters and increase the number of
inversions one at a time. After the m-th step of the process, we obtain a
random permutation s(n,m) that is uniformly distributed over all
permutations of {1,2,...,n} with m inversions. We will discuss the evolution
process, the connectedness threshold for the number of inversions of s(n,m),
and the sizes of the components when m is near the threshold value. This
study fits into the wider framework of random graphs since it is analogous
to studying phase transitions in random graphs. It is a joint work with my
adviser Boris Pittel.

Series: Combinatorics Seminar

This talk will be on an algebraic proof of theSzemeredi-Trotter theorem, as given by Kaplan, Matousek and Sharir.The lecture assumes no prior knowledge of advanced algebra.

Series: Combinatorics Seminar

The additive energy of a set of integers gives key
information on the additive structure of the set. In this talk, we
discuss a new, closely related statistic called the indexed additive
energy. We will investigate the relationship between the indexed
additive energy, the regular additive energy, and the size of the
sumset.

Series: Combinatorics Seminar

The Green-Tao theorem says that the primes contain arithmetic progressions
of arbitrary length. Tao and Ziegler extended it to polynomial
progressions, showing that congurations {a+P_1(d), ..., a+P_k(d)}
exist in the primes, where P_1, ..., P_k are polynomials in
\mathbf{Z}[x] without constant terms (thus the Green-Tao theorem
corresponds to the case where all the P_i are linear). We extend this
result further, showing that we can add the extra requirement that d be
of the form p-1 (or p + 1) where p is prime. This is joint work with
Julia Wolf.