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Toric Periods and $p$-adic Families of Modular Forms of Half-Integral Weight

About this Title

V. Vatsal

Publication: Memoirs of the American Mathematical Society
Publication Year: 2023; Volume 289, Number 1438
ISBNs: 978-1-4704-6550-6 (print); 978-1-4704-7593-2 (online)
DOI: https://doi.org/10.1090/memo/1438
Published electronically: August 24, 2023
Keywords: automorphic forms, metaplectic group, toric periods, test vectors

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Table of Contents

Chapters

  • Introduction

1. Part 1: Global periods: Fourier coefficients of half-integer weight forms

  • 1. Preliminaries
  • 2. Classical and automorphic forms of half-integer weight
  • 3. Fourier coefficients of modular forms of half-integer weight

2. Part 2: Interpolation of the Fourier coefficients

  • 4. Preliminaries and the basic formula
  • 5. $\widetilde \Lambda$-adic Waldspurger packets, and non-vanishing of the $\widetilde \Lambda$-adic lift.
  • 6. Specialization to trivial character, and the trivial zero

3. Part 3: Local periods: test vectors

  • 7. Preliminaries and statement of results
  • 8. $K$-types and the principal series
  • 9. The depth zero supercuspidal representations of $GL_2({\mathbf {Q}}_p)$.

Abstract

The primary goal of this work is to construct $p$-adic families of modular forms of half-integral weight, by using Waldspurger’s automorphic framework to make the results as comprehensive and precise as possible. A secondary goal is to clarify the role of test vectors as defined by Gross-Prasad in the elucidation of general formulae for the Fourier coefficients of modular forms of half-integral weight in terms of toric periods of the corresponding modular forms of integral weight. As a consequence of our work, we develop a generalization of a classical formula due to Shintani, and make precise the conditions under which Shintani’s lift vanishes. We also give a number of results on test vectors for ramified representations which are of independent interest.

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