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Arithmetic Quotients of the Bruhat-Tits Building for Projective General Linear Group in Positive Characteristic

About this Title

Satoshi Kondo and Seidai Yasuda

Publication: Memoirs of the American Mathematical Society
Publication Year: 2025; Volume 306, Number 1547
ISBNs: 978-1-4704-7313-6 (print); 978-1-4704-8055-4 (online)
DOI: https://doi.org/10.1090/memo/1547
Published electronically: January 27, 2025
Keywords: Modular symbol, automorphic representation, positive characteristic, Bruhat-Tits building

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Table of Contents

Chapters

  • 1. Introduction
  • 2. Generalized simplicial complexes and their (co)homology
  • 3. The Bruhat-Tits building and apartments
  • 4. Arithmetic subgroups and modular symbols
  • 5. Automorphic Forms with Steinberg at infinity
  • 6. Double cosets for automorphic forms
  • 7. Proof of Theorem
  • 8. Universal Modular Symbols
  • 9. On finite $p$-subgroups of arithmetic subgroups
  • 10. Some spectral sequences
  • 11. Proof for universal modular symbols
  • 12. Comparison of modular symbols

Abstract

Let $d \ge 1$. We study a subspace of the space of automorphic forms of $\mathrm {GL}_d$ over a global field of positive characteristic (or, a function field of a curve over a finite field). We fix a place $\infty$ of $F$, and we consider the subspace $\mathcal {A}_{\mathrm {St}}$ consisting of automorphic forms such that the local component at $\infty$ of the associated automorphic representation is the Steinberg representation (to be made precise in the text).

We have two results.

One theorem (Theorem 5.4.2) describes the constituents of $\mathcal {A}_{\mathrm {St}}$ as automorphic representation and gives a multiplicity one type statement.

For the other theorem (Theorem 4.5.1), we construct, using the geometry of the Bruhat-Tits building, an analogue of modular symbols in $\mathcal {A}_{\mathrm {St}}$ integrally (that is, in the space of $\mathbb {Z}$-valued automorphic forms). We show that the quotient is finite when a level is fixed and give a bound on the exponent of this quotient.

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