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summer 2000
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Wojciech J. Gajda

professor of mathematics

Mailing address:
Faculty of Mathematics and Computer Science
Adam Mickiewicz University
Umultowska 87, 61-614 PoznaƄ, POLAND
Office: Collegium Mathematicum, B1-35
Phone: +48 (61) 829 5503
Fax: +48 (61) 829 5315
Email: gajda AT amu DOT edu DOT pl

Research interests:

arithmetic geometry, number theory, algebraic topology, algebraic K-theory


Matija Kazalicki, University of Zagreb

30.01.2019 12:00, B1-37

Title: Congruences for sporadic sequences and modular forms for non-congruence subgroups

Abstract (PDF version):

In 1979, in the course of the proof of the irrationality of zeta(2) Robert Apery introduced numbers b_n = \sum_{k=0}^n {n \choose k}^2{n+k \choose k} that are, surprisingly, integral solutions of recursive relations (n+1)^2 u_{n+1} - (11n^2+11n+3)u_n-n^2u_{n-1} = 0. Zagier performed a computer search on first 100 million triples (A,B,C)\in Z^3 and found that the recursive relation generalizing b_n (n+1)u_{n+1} - (An^2+An+B)u_n + C n ^2 u_{n-1}=0, with the initial conditions u_{-1}=0 and u_0=1 has (non-degenerate i.e. C(A^2-4C)\ne 0) integral solution for only six more triples (whose solutions are so called sporadic sequences) . Stienstra and Beukers showed that the for prime p>=5 b_{(p-1)/2} \equiv \begin{cases} 4a^2-2p \pmod{p} \textrm{ if } p = a^2+b^2,\textrm{ a odd}\\ 0 \pmod{p} \textrm{ if } p\equiv 3 \pmod{4}.\end{cases} Recently, Osburn and Straub proved similar congruences for all but one of the six Zagier's sporadic sequences (three cases were already known to be true by the work of Stienstra and Beukers) and conjectured the congruence for the sixth sequence (which is a solution of recursion determined by triple (17,6,72). In this talk we prove that remaining congruence by studying Atkin and Swinnerton-Dyer congruences between Fourier coefficients of certain cusp form for non-congurence subgroup.