Let $k$ be a global field of positive characteristic, $\mathbb{A}$ its adele ring. Let $U$ be a unipotent algebraic group over $k$, of dimension sufficiently small relative to ${\rm char} (k)$. Is there a reference for the proof of a conjecture along the following lines? The space $L^2 (U(\mathbb{A}) / U(k))$, viewed as a $U(\mathbb{A})$-representation, decomposes into a direct sum of irreducibles, each with multiplicity one, parametrized by $\mathfrak{u} (k)^{*} / U(k)$ (in the spirit of the orbit method). Here $\mathfrak{u}$ is the Lie algebra of $U$.
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