Georgia Institute of Technology College of Sciences

TBA

Many problems in number theory boil down to bounding the size of a set contained in a certain set of residue classes mod $p$ for various sets of primes $p$; and then sieve methods are the primary tools for doing so. Motivated by the inverse Goldbach problem, Green–Harper, Helfgott–Venkatesh, Shao, and Walsh have explored the inverse sieve problem: if we let $S \subseteq [N]$ be a maximal set of integers in this interval where the residue classes mod $p$ occupied by $S$ have some particular pattern for many primes $p$, what can one say about the  structure of the set $S$ beyond just its size? In this talk, I will give a gentle introduction to inverse sieve problems, and present some progress we made when $S$ mod $p$ has rich additive structure for many primes $p$. In particular, in this setting, we provide several improvements on the larger sieve bound for $|S|$, parallel to the work of Green–Harper and Shao for improvements on the large sieve. Joint work with Ernie Croot and Junzhe Mao.

In this talk, we will discuss the construction of exotic 4-manifolds using Lefschetz fibrations over S^2, which are obtained by finite order cyclic group actions on Σg. We will first apply various cyclic group actions on Σg for g>0, and then extend it diagonally to the product manifolds ΣgxΣg. These will give singular manifolds with cyclic quotient singularities. Then, by resolving the singularities, we will obtain families of Lefschetz fibrations over S^2. Following the resolution process, we will determine the configurations of the singular fibers and the monodromy of the total space. In some cases, deformations of the Lefschetz fibrations give rise to nice applications using the rational blow-down operation, which provides exotic examples. This is a joint work with A. Akhmedov and M. Bhupal.

The degeneracy of a graph is a measure of sparseness that appears in many contexts throughout graph theory. In extremal graph theory, it is known that graphs of bounded degeneracy have Ramsey number which is linear in their number of vertices (Lee, 2017). Also, the degeneracy gives good bounds on the Turán exponent of bipartite graphs (Alon--Krivelevich--Sudokav, 2003). Extending these results to hypergraphs presents a challenge, as it is known that the naïve generalization of these results -- using the standard notion of hypergraph degeneracy -- are not true (Kostochka--Rödl 2006). We define a new measure of sparseness for hypergraphs called skeletal degeneracy and show that it gives information on both the Ramsey- and Turán-type properties of hypergraphs.

Based on joint work with Jacob Fox, Maya Sankar, Michael Simkin, and Yunkun Zhou