Isomorphic induced modules and Dynkin diagram automorphisms of semisimple Lie algebras

Abstract

Consider a simple Lie algebra $\mathfrak{g}$ and $\overline{\mathfrak{g}}% \subset \mathfrak{g}$ a Levi subalgebra. Two irreducible $\overline{% \mathfrak{g}}$-modules yield isomorphic inductions to $\mathfrak{g}$ when their highest weights coincide up to conjugation by an element of the Weyl group $W$ of $\mathfrak{g}$ which is also a Dynkin diagram automorphism of $% \overline{\mathfrak{g}}$. In this paper we study the converse problem: given two irreducible $\overline{\mathfrak{g}}$-modules of highest weight $\mu $ and $\nu $ whose inductions to $\mathfrak{g}$ are isomorphic, can we conclude that $\mu $ and $\nu $ are conjugate under the action of an element of $W$ which is also a Dynkin diagram automorphism of $\overline{\mathfrak{g}% }$ ? We conjecture this is true in general. We prove this conjecture in type $A$ and, for the other root systems, in various situations providing $\mu $ and $\nu $ satisfy additional hypotheses. Our result can be interpreted as an analogue for branching coefficient of the main result of \cite{Raj} on tensor product multiplicities.