Vol. 10, No. 8, 2016

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Steinberg groups as amalgams

Daniel Allcock

Vol. 10 (2016), No. 8, 1791–1843
Abstract

For any root system and any commutative ring, we give a relatively simple presentation of a group related to its Steinberg group St. This includes the case of infinite root systems used in Kac–Moody theory, for which the Steinberg group was defined by Tits and Morita–Rehmann. In most cases, our group equals St, giving a presentation with many advantages over the usual presentation of St. This equality holds for all spherical root systems, all irreducible affine root systems of rank > 2, and all 3-spherical root systems. When the coefficient ring satisfies a minor condition, the last condition can be relaxed to 2-sphericity.

Our presentation is defined in terms of the Dynkin diagram rather than the full root system. It is concrete, with no implicit coefficients or signs. It makes manifest the exceptional diagram automorphisms in characteristics 2 and 3, and their generalizations to Kac–Moody groups. And it is a Curtis–Tits style presentation: it is the direct limit of the groups coming from 1- and 2-node subdiagrams of the Dynkin diagram. Over nonfields this description as a direct limit is new and surprising. Our main application is that many Steinberg and Kac–Moody groups over finitely generated rings are finitely presented.

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Keywords
Kac–Moody group, Steinberg group, pre-Steinberg group, Curtis–Tits presentation
Mathematical Subject Classification 2010
Primary: 19C99
Secondary: 20G44, 14L15
Milestones
Received: 29 March 2016
Accepted: 11 June 2016
Published: 7 October 2016
Authors
Daniel Allcock
Department of Mathematics
University of Texas at Austin
RLM 8.100
2515 Speedway Stop C1200
Austin, TX 78712
United States