Romik's conjecture for the Jacobi theta function

Dan Romik recently considered the Taylor coefficients of the Jacobi theta function around the complex multiplication point i. He then conjectured that the Taylor coefficients d(n) either vanish or are periodic modulo any prime p; this was proved by the combined efforts of Scherer and Guerzhoy-Mertens-Rolen, with the latter trio considering arbitrary half integral weight modular forms. We refine previous work for p 1 (mod 4) by displaying a concise algebraic relation between d (n + p-1/2) related to the p-adic factorial, from which we can deduce periodicity with an effective period. (C) 2020 Elsevier Inc. All rights reserved.