A proof of the Zariski closure conjecture and quasi-Frobenius exponential algebras

Abstract

Let $G=\exp(\g)$ be an exponential solvable Lie group, $π\in\widehat G$, and $Ω_π$ the coadjoint orbit attached to $π$ by the orbit method. The Zariski closure conjecture asserts that the Poisson characteristic variety $\VA(π_ν)$ of the deformed representation coincides with the Zariski closure of $Ω_π$ in $\g^*$. Known until now only in special situations, including the nilpotent case, it is proved here in full generality. Since the Hilbert space of $π$ is a quantization of the symplectic manifold $Ω_π$, the statement says that the passage back from the representation to the orbit, a dequantization, loses nothing: the orbit is recovered, up to Zariski closure, from the algebra alone. We also characterize the quasi-Frobenius exponential Lie algebras, those whose dual vector space is a Zariski-dense coadjoint orbit, through the triviality of the annihilator of an irreducible representation.