Invariants and K-spectrums of local theta lifts

Abstract

Let $(G,G')$ be a type I irreducible reductive dual pair in $\mathrm{Sp}(W_{\mathbb{R}})$. We assume that $(G,G')$ is in the stable range where $G$ is the smaller member. Let $K$ and $K'$ be maximal compact subgroups of $G$ and $G'$ respectively. Let $\mathfrak{g} = \mathfrak{k} \oplus \mathfrak{p}$ and $\mathfrak{g}' = \mathfrak{k}' \oplus \mathfrak{p}'$ be the complexified Cartan decompositions of the Lie algebras of $G$ and $G'$ respectively. Let ${\widetilde{K}}$ and ${\widetilde{K}}'$ be the inverse images of $K$ and $K'$ in the metaplectic double cover $\widetilde{\mathrm{Sp}}(W_\mathbb{R})$ of ${\mathrm{Sp}}(W_\mathbb{R})$. Let $ρ$ be a genuine irreducible $(\mathfrak{g},{\widetilde{K}})$-module. Our first main result is that if $ρ$ is unitarizable, then except for one special case, the full local theta lift $ρ' = Θ(ρ)$ is equal to the local theta lift $θ(ρ)$. Thus excluding the special case, the full theta lift $ρ'$ is an irreducible and unitarizable $(\mathfrak{g}',{\widetilde{K}}')$-module. Our second main result is that the associated variety and the associated cycle of $ρ'$ are the theta lifts of the associated variety and the associated cycle of the contragredient representation $ρ^*$ respectively. Finally we obtain some interesting $(\mathfrak{g},{\widetilde{K}})$-modules whose ${\widetilde{K}}$-spectrums are isomorphic to the spaces of global sections of some vector bundles on some nilpotent $K_\mathbb{C}$-orbits in $\mathfrak{p}^*$.