NTS ABSTRACTFall2025

Back to the number theory seminar main webpage: Main page

Sep 11

Enumerating Galois extensions of number fields

Robert Lemke Oliver (UW-Madison)

We provide an asymptotic formula for the number of Galois extensions of a number field with absolute discriminant bounded by some X.  The key behind this result is a new upper bound on the number of Galois extensions with a given Galois group G of order at least 5.  In particular, we give the first general bound with an exponent that decays with the order of G.  This improves over the previous best bound due to Ellenberg and Venkatesh.

Sep 18

Isogenies between reductions of elliptic curves with complex multiplication

Jiacheng Xia (UW Madison)

Given two elliptic curves over a number field, Charles proved that there are infinitely many primes where the reductions of these two curves are geometrically isogenous. We study a refined problem for a given pair of CM elliptic curves $E_1$ and $E_2$: for a positive integer $m$, how many primes are there where the reductions of $E_1$ and $E_2$ are related by a cyclic isogeny of degree $m$? We establish a polynomial lower bound in $m$ for such a counting problem. Gross-Zagier type theorems and higher Green functions connect our counting problem to the Fourier coefficients of certain incoherent Eisenstein series, which can in turn be approximated by those of certain elliptic cusp forms which are not necessarily eigenforms. One further step is to establish an explicit Deligne bound for a general cusp form of arbitrary weight and level, which might be novel and of independent interest. This is a joint work with Edgar Assing, Yingkun Li, and Tian Wang.

Oct 16

Qiao He (Columbia)

Oct 30

Beilinson-Bloch-Kato conjecture for polarized motives

Hao Peng (MIT)

The Beilinson—Bloch—Kato conjecture is a far-fetching generalization of the (rank part of the) BSD conjecture for modular elliptic curves. The conjecture is partially proved for U(N)*U(N+1)-motives in the work of Y. Liu, Y. Tian, L. Xiao, W. Zhang, and X. Zhu. Using theta correspondence, we prove that their result implies the BBK conjecture for U(2n)-motives, e.g. odd symmetric powers of non-CM modular elliptic curves, in the rank zero case. Similar trick works in the orthogonal case. If time permits, we talk about the work in progress partiallu proving the BBK conjecture for O(N)*O(N+1)-motives when analytic rank is at most one.