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The Deligne–Mostow $9$-ball, and the monster

Daniel Allcock and Tathagata Basak

Geometry & Topology 29 (2025) 791–828
Abstract

The “monstrous proposal” of the first author is that the quotient of a certain 13-dimensional complex hyperbolic braid group, by the relations that its natural generators have order 2, is the “bimonster” (M × M) 2. Here M is the monster simple group. We prove that this quotient is either the bimonster or 2. In the process, we give new information about the isomorphism, found by Deligne and Mostow, between the moduli space of 12-tuples in P1 and a quotient of the complex 9-ball. Namely, we identify which loops in the 9-ball quotient correspond to the standard braid generators.

Keywords
monster, ball quotient, braid group
Mathematical Subject Classification
Primary: 20F36, 22E40
Secondary: 20D08
References
Publication
Received: 31 August 2023
Revised: 25 March 2024
Accepted: 4 May 2024
Published: 12 March 2025
Proposed: Benson Farb
Seconded: David Fisher, Mladen Bestvina
Authors
Daniel Allcock
Department of Mathematics
University of Texas at Austin
Austin, TX
United States
http://www.math.utexas.edu/~allcock
Tathagata Basak
Department of Mathematics
Iowa State University
Ames, IA
United States
https://orion.math.iastate.edu/tathagat/

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