A characterization of unitarity of some highest weight Harish-Chandra modules

Abstract

Let $L(λ)$ be a highest weight Harish-Chandra module with highest weight $λ$. When the associated variety of $L(λ)$ is not maximal, that is, not equal to the nilradical of the corresponding parabolic subalgebra, we prove that the unitarity of $L(λ)$ can be determined by a simple condition on the value of $z = (λ+ ρ, β^{\vee})$, where $ρ$ is half the sum of positive roots and $β$ is the highest root. In the proof, certain distinguished antichains of positive noncompact roots play a key role. By using these antichains, we are also able to provide a uniform formula for the Gelfand--Kirillov dimension of all highest weight Harish-Chandra modules, generalizing our previous result for the case of unitary highest weight Harish-Chandra modules.