Admissibility For Quasiregular Representations of Exponential Solvable Lie Groups

Let $N$ be a simply connected, connected non-commutative nilpotent Lie group with Lie algebra $\mathfrak{n}$ of dimension $n.$ Let $H$ be a subgroup of the automorphism group of $N.$ Assume that $H$ is a commutative, simply connected, connected Lie group with Lie algebra $\mathfrak{h}.$ Furthermore, let us assume that the linear adjoint action of $\mathfrak{h}$ on $\mathfrak{n}$ is diagonalizable with non-purely imaginary eigenvalues. Let $\tau=\mathrm{Ind}$. We obtain an explicit direct integral decomposition for $\tau$, including a description of the spectrum as a sub-manifold of $(\mathfrak{n}+\mathfrak{h})^{\ast}$, a formula for the multiplicity function of the unitary irreducible representations occurring in the direct integral, and a precise intertwining operator. Finally, we completely settle the admissibility question of $\tau$. In fact, we show that if $G=N\rtimes H$ is unimodular, then $\tau$ is never admissible, and if $G$ is nonunimodular, $\tau$ is admissible if and only if the intersection of $H$ and the center of $G$ is equal to the identity of the group.