Let $E/F$ be a quadratic extension of number fields, and let $V$ be an $n$-dimensional Hermitian space over $E$.


Let $\tilde{G} := GU(V)$ and $G := U(V)$. Suppose that $(\pi, V\_{\pi})$ is an irreducible cuspidal representation of $G.$


Is there an irreducible cuspidal representation $(\tilde{\pi}, V\_{\tilde{\pi}})$ of $\tilde{G}$ such that $V\_\pi \subset V\_{\tilde{\pi}}|\_{G}$? Note that here, the restriction is that of cusp forms, not of the representation itself.



extending cusp forms
I believe the answer should be yes, by some version of the following sketch of an argument:


(Note: by restriction of scalars, I regard all groups as being defined over $\mathbb Q$,
and I write ${\mathbb A}$ for the adeles of $\mathbb Q$.)


We are given $V\_{\pi} \subset Cusp(G(F)\backslash G({\mathbb A}\_F)).$


Let $\tilde{C}$ denote the maximal $\mathbb Q$-split torus in the centre of $\tilde{G}$
(this is just a copy of $\mathbb G\_m$), and write $C = \tilde{C}\cap G$. (I guess
this is just $\pm 1$?)


Now $C(\mathbb A)$ acts on $V\_{\pi}$ through some character $\chi$ of $(\mathbb A)/C(\mathbb Q)$. Choose an extension
$\tilde{\chi}$ of $\chi$ to a character of $\tilde{C}(\mathbb A)/\tilde{C}(\mathbb Q)$, 
and regard $V\_{\pi}$ as a representation of $\tilde{C} G$ by have $\tilde{C}$ act
through $\tilde{\chi}$. Since $\tilde{C} G$ is normal and Zariksi open in $\tilde{G}$,
we should be able to further extend the $\tilde{C} G(\mathbb A)$-action on $V\_{\pi}$
to an action of $\tilde{G}(\mathbb Q)\tilde{C}G(\mathbb A).$


Now if we consider $Ind\_{\tilde{G}(\mathbb Q)\tilde{C} G(\mathbb A)}^{\tilde{G}(\mathbb A)} V\_{\pi},$ we should be able to
find a cupsidal representation $V\_{\tilde{\pi}}$ of the form you want (with $\tilde{C}(A)$ acting via $\tilde{\chi}$).


The intuition is that automorphic forms on $G(\mathbb A)$ are $Ind\_{G(\mathbb Q)}^{G(\mathbb A)} 1,$
and similarly for $\tilde{G}$. We will consider variants of this formula that takes into account central characters, and think about how to compare them for $G$ and $\tilde{G}$.


Inside the automorphic forms, we have those where $C(\mathbb A)$ acts by $\chi$; this we can
write as $Ind\_{G(\mathbb Q)C(\mathbb A)}^{G(\mathbb A)} \chi$, and then rewrite as
$Ind\_{\tilde{G}(\mathbb Q)\tilde{C}(\mathbb A)}^{\tilde{G}(\mathbb Q)\tilde{C}G(\mathbb A)} \tilde{\chi}.$ This is where $V\_{\pi}$ lives, once we extend it to a repreresentation
of $\tilde{G}(\mathbb Q)\tilde{C}G(\mathbb A)$ as above.


Now the automorphic forms on $\tilde{G}(\mathbb A)$ with central character $\tilde{\chi}$
are $Ind\_{\tilde{G}(\mathbb Q)\tilde{C}(\mathbb A)}^{\tilde{G}(\mathbb A)} \tilde{\chi},$
which we can rewrite as
$Ind\_{\tilde{G}(\mathbb Q) \tilde{C}G(\mathbb A)}^{\tilde{G}(\mathbb A)}
Ind\_{\tilde{G}(\mathbb Q)\tilde{C}(\mathbb A)}^{\tilde{G}(\mathbb Q)\tilde{C}G(\mathbb A)}
\tilde{\chi}.$ This thus contains $Ind\_{\tilde{G}(\mathbb Q)\tilde{C}G(\mathbb A)}^{\tilde{G}(\mathbb A)}V\_{\pi}$ inside it, and so an irreducible constituent of the latter
should be a $V\_{\tilde{\pi}}$ whose restriction (as a space of functions) to $G(\mathbb A)$ contains $V\_{\pi}$. 


What I have just discussed is the analogue for $G$ and $\tilde{G}$ of the relation between automorphic forms on $SL\_2$ and $GL\_2$ discussed e.g. in Langlands--Labesse. Hopefully I haven't blundered!


