A Geometric Approach to Complete Reducibility

Details A Geometric Approach to Complete Reducibility A Geometric Approach to Complete Reducibility

2004-08-09

arxiv.org/abs/math/0408109v2

Mathematics

Group Theory

35 pages

Scientific paper

10.1007/s00222-004-0425-9

Let G be a connected reductive linear algebraic group. We use geometric methods to investigate G-completely reducible subgroups of G, giving new criteria for G-complete reducibility. We show that a subgroup of G is G-completely reducible if and only if it is strongly reductive in G; this allows us to use ideas of R.W. Richardson and Hilbert--Mumford--Kempf from geometric invariant theory. We deduce that a normal subgroup of a G-completely reducible subgroup of G is again G-completely reducible, thereby providing an affirmative answer to a question posed by J.-P. Serre, and conversely we prove that the normalizer of a G-completely reducible subgroup of G is again G-completely reducible. Some rationality questions and applications to the spherical building of G are considered. Many of our results extend to the case of non-connected G.

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