  Let $F$ be a $p$-adic field and $\mathbf{U}$ be a unipotent group defined
over $F$, and set $U=\mathbf{U}(F)$. Let $\sigma$ be an involution of
$\mathbf{U}$ defined over $F$. Adapting the arguments of Yves Benoist in the
real case, we prove the following result: an irreducible representation $\pi$
of $U$ is $U^{\sigma}$-distinguished if and only if it is $\sigma$-self-dual
and in this case $\mathrm{Hom}_{U^\sigma}(\pi,\mathbb{C})$ has dimension one.
When $\sigma$ is a Galois involution these results imply a bijective
correspondence between the set $\mathrm{Irr}(U^\sigma)$ of isomorphism classes
of irreducible representations of $U^\sigma$ and the set
$\mathrm{Irr}_{U^\sigma-\mathrm{dist}}(U)$ of isomorphism classes of
distinguished irreducible representations of $U$.

Representation Theory