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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.

7 votes
2 answers
2k views

When is the Levi subalgebra an ideal?

Let $L$ be a Lie algebra. It is known that $L$ admits a Levi decomposition (possibly non unique): $L=S\oplus rad(L) $, where $rad(L)$ is the solvable radical and $S$ is a semisimple subalgebra. If …
10 votes
2 answers
1k views

Abstract Jordan Decomposition different from usual Jordan Decomposition

It's known that if $L\subset gl(V)$, with $V$ finite dimensional, is a semisimple Lie algebra, then the abstract and usual Jordan decompositions in $L$ coincide. Is it possible to provide a counter-ex …
5 votes
2 answers
315 views

Radical of projection equals projection of radical?

Given an Lie Algebra $L$ (of finite dimension and over an algebraically closed field with zero characteristic) and an ideal $I$, is it truth that $rad\left(\dfrac{L}{I}\right)= \pi(rad(L))$, where …