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Title: On unipotent supports of reductive groups with a disconnected centre
Author: Taylor, Jonathan
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2012
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Let G be a connected reductive algebraic group defined over an algebraic closure of the finite field of prime order p > 0, which we assume to be good for G. We denote by F : G → G a Frobenius endomorphism of G and by G the corresponding Fq-rational structure. If Irr(G) denotes the set of ordinary irreducible characters of G then by work of Lusztig and Geck we have a well defined map ΦG : Irr(G) → {F-stable unipotent conjugacy classes of G} where ΦG(χ) is the unipotent support of χ. Lusztig has given a classification of the irreducible characters of G and obtained their degrees. In particular he has shown that for each χ ∈ Irr(G) there exists an integer nχ such that nχ · χ(1) is a monic polynomial in q. Given a unipotent class O of G with representative u ∈ G we may define AG(u) to be the finite quotient group CG(u)/CG(u)◦. If the centre Z(G) is connected and G/Z(G) is simple then Lusztig and H´ezard have independently shown that for each F-stable unipotent class O of G there exists χ ∈ Irr(G) such that ΦG(χ) = O and nχ = |AG(u)|, (in particular the map ΦG is surjective). The main result of this thesis extends this result to the case where G is any simple algebraic group, (hence removing the assumption that Z(G) is connected). In particular if G is simple we show that for each F-stable unipotent class O of G there exists χ ∈ Irr(G) such that ΦG(χ) = O and nχ = |AG(u)F| where u ∈ OF is a well-chosen representative. We then apply this result to prove, (for most simple groups), a conjecture of Kawanaka’s on generalised Gelfand–Graev representations (GGGRs). Namely that the GGGRs of G form a Z-basis for the Z-module of all unipotently supported class functions of G. Finally we obtain an expression for a certain fourth root of unity associated to GGGRs in the case where G is a symplectic or special orthogonal group.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Finite groups