English

Structure of an exotic $2$-local subgroup in $E_7(q)$

Group Theory 2026-05-12 v2

Abstract

Let GG be the finite simple group of Lie type G=E7(q)G = E_7(q), where qq is an odd prime power. Then GG is an index 22 subgroup of the adjoint group GadG_{\operatorname{ad}}, which is also denoted by Gad=Inndiag(G)G_{\operatorname{ad}} = \operatorname{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 22-subgroup EE of order 44 in GadG_{\operatorname{ad}}, such that NGad(E)/CGad(E)Sym3N_{G_{\operatorname{ad}}}(E)/C_{G_{ad}}(E) \cong \operatorname{Sym}_3, and CGad(E)=E×Inndiag(D4(q))C_{G_{\operatorname{ad}}}(E) = E \times \operatorname{Inndiag}(D_4(q)). Furthermore, such an EE is unique up to conjugacy in GadG_{\operatorname{ad}}. It is known that NG(E)N_G(E) is always a maximal subgroup of GG, and NGad(E)N_{G_{\operatorname{ad}}}(E) is a maximal subgroup of GadG_{\operatorname{ad}} unless NGad(E)GN_{G_{\operatorname{ad}}}(E) \leq G. In this note, we describe the structure of NG(E)N_{G}(E). It turns out that NG(E)=NGad(E)N_G(E) = N_{G_{\operatorname{ad}}}(E) if and only if q±1mod8q \equiv \pm 1 \mod{8}.

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Cite

@article{arxiv.2409.20281,
  title  = {Structure of an exotic $2$-local subgroup in $E_7(q)$},
  author = {Mikko Korhonen},
  journal= {arXiv preprint arXiv:2409.20281},
  year   = {2026}
}

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9 pages