# Colloquia/Fall18

## Contents

# Mathematics Colloquium

All colloquia are on Fridays at 4:00 pm in Van Vleck B239, **unless otherwise indicated**.

## Spring 2018

date | speaker | title | host(s) | |
---|---|---|---|---|

January 29 (Monday) | Li Chao (Columbia) | Elliptic curves and Goldfeld's conjecture | Jordan Ellenberg | |

February 2 | Thomas Fai (Harvard) | TBA | Spagnolie, Smith | |

February 5 (Monday) | Alex Lubotzky (Hebrew University) | High dimensional expanders: From Ramanujan graphs to Ramanujan complexes | Ellenberg, Gurevitch | |

February 6 (Tuesday 2 pm) | Alex Lubotzky (Hebrew University) | Groups' approximation, stability and high dimensional expanders | Ellenberg, Gurevitch | |

February 9 | Wes Pegden (CMU) | TBA | Roch | |

March 16 | Anne Gelb (Dartmouth) | TBA | WIMAW | |

April 4 (Wednesday) | John Baez (UC Riverside) | TBA | Craciun | |

April 6 | Reserved | TBA | Melanie | |

April 13 | Jill Pipher (Brown) | TBA | WIMAW | |

April 25 (Wednesday) | Hitoshi Ishii (Waseda University) Wasow lecture | TBA | Tran | |

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## Spring Abstracts

### January 29 Li Chao (Columbia)

Title: Elliptic curves and Goldfeld's conjecture

Abstract: An elliptic curve is a plane curve defined by a cubic equation. Determining whether such an equation has infinitely many rational solutions has been a central problem in number theory for centuries, which lead to the celebrated conjecture of Birch and Swinnerton-Dyer. Within a family of elliptic curves (such as the Mordell curve family y^2=x^3-d), a conjecture of Goldfeld further predicts that there should be infinitely many rational solutions exactly half of the time. We will start with a history of this problem, discuss our recent work (with D. Kriz) towards Goldfeld's conjecture and illustrate the key ideas and ingredients behind these new progresses.

### February 5 Alex Lubotzky (Hebrew University)

Title: High dimensional expanders: From Ramanujan graphs to Ramanujan complexes

Abstract:

Expander graphs in general, and Ramanujan graphs , in particular, have played a major role in computer science in the last 5 decades and more recently also in pure math. The first explicit construction of bounded degree expanding graphs was given by Margulis in the early 70's. In mid 80' Margulis and Lubotzky-Phillips-Sarnak provided Ramanujan graphs which are optimal such expanders.

In recent years a high dimensional theory of expanders is emerging. A notion of topological expanders was defined by Gromov in 2010 who proved that the complete d-dimensional simplical complexes are such. He raised the basic question of existence of such bounded degree complexes of dimension d>1.

This question was answered recently affirmatively (by T. Kaufman, D. Kazdhan and A. Lubotzky for d=2 and by S. Evra and T. Kaufman for general d) by showing that the d-skeleton of (d+1)-dimensional Ramanujan complexes provide such topological expanders. We will describe these developments and the general area of high dimensional expanders.

### February 6 Alex Lubotzky (Hebrew University)

Title: Groups' approximation, stability and high dimensional expanders

Abstract:

Several well-known open questions, such as: are all groups sofic or hyperlinear?, have a common form: can all groups be approximated by asymptotic homomorphisms into the symmetric groups Sym(n) (in the sofic case) or the unitary groups U(n) (in the hyperlinear case)? In the case of U(n), the question can be asked with respect to different metrics and norms. We answer, for the first time, one of these versions, showing that there exist fintely presented groups which are not approximated by U(n) with respect to the Frobenius (=L_2) norm.

The strategy is via the notion of "stability": some higher dimensional cohomology vanishing phenomena is proven to imply stability and using high dimensional expanders, it is shown that some non-residually finite groups (central extensions of some lattices in p-adic Lie groups) are Frobenious stable and hence cannot be Frobenius approximated.

All notions will be explained. Joint work with M, De Chiffre, L. Glebsky and A. Thom.