Department of

# Mathematics

Seminar Calendar
for events the day of Tuesday, February 12, 2019.

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events for the
events containing

Questions regarding events or the calendar should be directed to Tori Corkery.
     January 2019          February 2019            March 2019
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa
1  2  3  4  5                   1  2                   1  2
6  7  8  9 10 11 12    3  4  5  6  7  8  9    3  4  5  6  7  8  9
13 14 15 16 17 18 19   10 11 12 13 14 15 16   10 11 12 13 14 15 16
20 21 22 23 24 25 26   17 18 19 20 21 22 23   17 18 19 20 21 22 23
27 28 29 30 31         24 25 26 27 28         24 25 26 27 28 29 30
31


Tuesday, February 12, 2019

1:00 pm in 345 Altgeld Hall,Tuesday, February 12, 2019

#### The Open Graph Dichotomy and the second level of the Borel hierarchy

###### Raphaël Carroy (Gödel Research Center for Math. Logic at Univ. of Vienna)

Abstract: I will explain how variants of the open graph dichotomy can be used to obtain various descriptive-set-theoretical dichotomies at the second level of the Borel hierarchy. This shows how to generalise these dichotomies from analytic metric spaces to separable metric spaces by working under the axiom of determinacy. If time allows it, I will also discuss some connections between cardinal invariants and the chromatic number of the graphs at stake.

2:00 pm in 243 Altgeld Hall,Tuesday, February 12, 2019

#### On the number of edges in C_5-free 3-uniform hypergraphs

###### Dara Zirlin (Illinois Math)

Abstract: In a 3-uniform hypergraph, a Berge 5-cycle is formed by five distinct edges $e_1,\dots e_5$ and five distinct vertices $v_1,\dots, v_5$, such that $v_i,v_{i+1}\in e_i$, where indices count modulo 5.

In 2007, Bollobás and Györi gave upper bounds on the number of triangles in a $C_5$-free graph, and on the number of edges in a 3-uniform hypergraph containing no Berge 5-cycles.
We improve their second bound. This is joint work with Alexandr Kostochka.

4:00 pm in Altgeld Hall,Tuesday, February 12, 2019

#### No Seminar Today

Abstract: To encourage faculty members of the seminar to join the 4pm departmental discussion in 245 of the math building design we won't have a seminar today. It will resume next week at the usual time and location.

4:00 pm in 245 Altgeld Hall,Tuesday, February 12, 2019

#### Altgeld-Illini Renovation/Building Project Feedback Session

Abstract: The department's Altgeld-Illini Renovation Committee seeks input from every member of the department to help us develop a clear vision of what we want in the new building and the renovations.