Nonholomorphic Ramanujan-type congruences for Hurwitz class numbers.

Beckwith, Olivia; Raum, Martin; Richter, Olav K · Proc Natl Acad Sci U S A · 2020

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Abstract

In contrast to all other known Ramanujan-type congruences, we discover that Ramanujan-type congruences for Hurwitz class numbers can be supported on nonholomorphic generating series. We establish a divisibility result for such nonholomorphic congruences of Hurwitz class numbers. The two key tools in our proof are the holomorphic projection of products of theta series with a Hurwitz class number generating series and a theorem by Serre, which allows us to rule out certain congruences.