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Structure of an exotic 2-local subgroup in 𝐸7(π‘ž)

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Published/Copyright: May 27, 2025

Abstract

Let 𝐺 be the finite simple group of Lie type G = E 7 ⁒ ( q ) , where π‘ž is an odd prime power. Then 𝐺 is an index 2 subgroup of the adjoint group G ad , which is also denoted by G ad = Inndiag ⁑ ( G ) and known as the group of inner-diagonal automorphisms. It was proven by Cohen–Liebeck–Saxl–Seitz (1992) that there is an elementary abelian 2-subgroup 𝐸 of order 4 in G ad such that

N G ad ⁒ ( E ) / C G ad ⁒ ( E ) β‰… Sym 3 and C G ad ⁒ ( E ) = E Γ— Inndiag ⁑ ( D 4 ⁒ ( q ) ) .

Furthermore, such an 𝐸 is unique up to conjugacy in G ad . It is known that N G ⁒ ( E ) is always a maximal subgroup of 𝐺, and N G ad ⁒ ( E ) is a maximal subgroup of G ad unless N G ad ⁒ ( E ) ≀ G . In this note, we describe the structure of N G ⁒ ( E ) . It turns out that we have N G ⁒ ( E ) = N G ad ⁒ ( E ) if and only if q ≑ Β± 1 mod 8 .

Award Identifier / Grant number: 12350410360

Funding statement: Supported by NSFC grant 12350410360.

Acknowledgements

The author would like to thank David Craven and Donna Testerman for helpful discussions and comments, and an anonymous referee for pointing out some misprints in an earlier version of this paper.

  1. Communicated by: Christopher W. Parker

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Received: 2024-12-27
Revised: 2025-04-27
Published Online: 2025-05-27
Published in Print: 2025-11-01

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