Complete reducibility for Lie subalgebras and semisimplification

Abstract

AbstractLet G be a connected reductive linear algebraic group over a field k. Using ideas from geometric invariant theory, we study the notion of G-complete reducibility over k for a Lie subalgebra $${\mathfrak {h}}$$ h of the Lie algebra $${\mathfrak {g}} = \hbox {Lie} (G)$$ g = Lie ( G ) of G and prove some results when $${\mathfrak {h}}$$ h is solvable or $$\textrm{char}\hspace{0.55542pt}(k)= 0$$ char ( k ) = 0 . We introduce the concept of a k-semisimplification$${\mathfrak {h}}'$$ h ′ of $${\mathfrak {h}}$$ h ; $${\mathfrak {h}}'$$ h ′ is a Lie subalgebra of $${\mathfrak {g}}$$ g associated to $${\mathfrak {h}}$$ h which is G-completely reducible over k. This is the Lie algebra counterpart of the analogous notion for subgroups studied earlier by the first, third and fourth authors. As in the subgroup case, we show that $${\mathfrak {h}}'$$ h ′ is unique up to $$\textrm{Ad}\hspace{0.55542pt}(G(k))$$ Ad ( G ( k ) ) -conjugacy in $${\mathfrak {g}}$$ g . Moreover, we prove that the two concepts are compatible: for H a closed subgroup of G and $$H'$$ H ′ a k-semisimplification of H, the Lie algebra $$\textrm{Lie}\hspace{0.55542pt}(H')$$ Lie ( H ′ ) is a k-semisimplification of $$\textrm{Lie}\hspace{0.55542pt}(H)$$ Lie ( H ) .