Wide Subalgebras of Semisimple Lie Algebras

Let G be a connected semisimple algebraic group over \({\mathbb C}\) , with Lie algebra \({\mathfrak g}\) . Let \({\mathfrak h}\) be a subalgebra of \({\mathfrak g}\) . A simple finite-dimensional \({\mathfrak g}\) -module \({\mathbb V}\) is said to be \({\mathfrak h}\) -indecomposable if it cannot be written as a direct sum of two proper \({\mathfrak h}\) -submodules. We say that \({\mathfrak h}\) is wide, if all simple finite-dimensional \({\mathfrak g}\) -modules are \({\mathfrak h}\) -indecomposable. Some very special examples of indecomposable modules and wide subalgebras appear recently in the literature. In this paper, we describe several large classes of wide subalgebras of \({\mathfrak g}\) and initiate their systematic study. Our approach is based on the study of idempotents in the associative algebra of \({\mathfrak h}\) -invariant endomorphisms of \({\mathbb V}\) . We also discuss a relationship between wide subalgebras and epimorphic subgroups.