Department of

# Mathematics

Seminar Calendar
for events the day of Tuesday, September 29, 2015.

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events for the
events containing

Questions regarding events or the calendar should be directed to Tori Corkery.
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Tuesday, September 29, 2015

11:00 am in 241 Altgeld Hall,Tuesday, September 29, 2015

#### Exponential sums over smooth numbers

###### Adam Harper (University of Cambridge)

Abstract: A number is said to be $y$-smooth if all of its prime factors are at most $y$. Exponential sums over the $y$-smooth numbers less than $x$ have been widely investigated, but existing results were weak for $y$ too small compared with $x$. For example, if $y$ is a power of $\log x$ then existing results were insufficient to study ternary additive problems involving smooth numbers, except by assuming conjectures like the Generalised Riemann Hypothesis. I will try to describe my work on bounding mean values of exponential sums over smooth numbers, which allows an unconditional treatment of ternary additive problems even with $y$ a (large) power of $\log x$. There are connections with restriction theory and additive combinatorics.

12:00 pm in 345 Altgeld Hall,Tuesday, September 29, 2015

#### Finding paths in graphs of triangulations

###### Mark Bell (UIUC Math)

Abstract: There are many different ways of triangulating a surface using n arcs. As some triangulations are more similar than others, we get a natural topology on the space of triangulations which can be seen as an infinite graph; where two triangulations are connected if and only if they share (n-1) edges. This graph provides a combinatorial model for the surfaces mapping class group as it acts geometrically. We will look at some techniques for efficiently finding paths through this graph, allowing us to efficiently represent and compute with mapping classes.

1:00 pm in 345 Altgeld Hall,Tuesday, September 29, 2015

#### Analytic sets defined by the metric in Polish/Banach spaces

###### Robert Kaufman   [email] (UIUC Math)

Abstract: We study analytic sets whose definition reflects exact choice of a metric, e.g., sets defined by best approximation. A pair $(a,b)$ is "balanced (bisected? split? dyadic?) if some element bisects the pair. The set B of balanced pairs in a Polish space is sometimes non-Borel; the proof yields a much more precise statement.
A set $S$ in a Banach space $M$ is "centered" if $S$ admits a best approximation by singletons in the Hausdorff metric. The remaining results depend on the notions of re-norming, dual space, reflexive space [in order of difficulty]. In dual spaces, all sets of finite diameter are centered. (Converse false) The following result is proved for the space $\ell^1$ by elementary methods: if $M$ is non-reflexive (NR) then it can be re-normed so that some tripleton isn't centered. When $M$ is separable, the set of centered tripletons is analytic; if $M$ is also NR, then that set becomes non-Borel after a re-norming.

1:00 pm in 241 Altgeld Hall,Tuesday, September 29, 2015

#### The Bergman kernel on some Reinhardt domains

###### Zhenghui Huo (UIUC Math)

2:00 pm in 347 Altgeld Hall,Tuesday, September 29, 2015

#### Trace and Weyl asymptotics for non-local operators

###### Selma Yildirim Yolcu   [email] (Bradley University)

Abstract: I will present a broad overview of exciting problems and conjectures which lie at the interface of probability and the spectral theory. In particular, I will discuss a certain problem of computing the coefficients in the two-term asymptotic formula for the trace of some Schrodinger operators. After recalling some of the well-known results, I will discuss recent (and not so recent) progress in understanding the analogous picture when the Laplacian, which "goes" with the Brownian motion, is replaced by non-local operators which "go" with rotationally invariant symmetric stable processes and other closely related Levy processes.

3:00 pm in 241 Altgeld Hall,Tuesday, September 29, 2015

#### Ordered Ramsey Theory and Track Representations of Graphs

###### Douglas B. West   [email] (Zhejiang Normal University and UIUC Math)

Abstract: We study an ordered version of hypergraph Ramsey numbers for linearly ordered vertex sets, due to Fox, Pach, Sudakov, and Suk. In the $k$-uniform ordered path, the edges are the sets of $k$ consecutive vertices in a linear order. Moshkovitz and Shapira described its ordered Ramsey number in terms of an enumerative problem: it equals $1$ plus the number of elements in the poset obtained by starting with a certain disjoint union of chains and repeatedly taking the poset of down-sets, $k-1$ times. We will describe a proof of this and apply the bounds to study the minimum number of interval graphs whose union is the line graph of the $n$-vertex complete graph. We prove the conjecture of Heldt, Knauer, and Ueckerdt that this value grows with $n$. In fact, the growth rate is between $\Omega({\log\log n}/{\log\log\log n})$ and $O(\log\log n)$. This work is joint with Kevin Milans and Derrick Stolee.

4:00 pm in 245 Altgeld Hall,Tuesday, September 29, 2015

#### Life After Comps

Abstract: The panelists will be math graduate students who are near or past the prelim, and the event is meant to benefit graduate students who are at the stage of choosing a thesis adviser and thesis topic, or earlier. Come and listen, and ask whatever questions you’d like, or share your experience and advice, whether you are a panelist or not.