Let $E / F$ be a quadratic extension of nonarchimedean local fields (characteristic 0 if it matters), and $\pi$ an irreducible infinite-dimensional smooth representation of $GL\_2(E)$. Let $B$ be the upper-triangular Borel of $GL\_2$. I'd like to know: do we always have
$$\operatorname{dim} Hom\_{B(F)}(\pi, \mathbf{C}) = 1 ?$$
I know how to construct a non-zero element in this space, so my concern is to prove its dimension is $\le 1$, i.e. to show that $(GL\_2(E), B(F))$ is a [Gelfand pair](https://en.wikipedia.org/wiki/Gelfand_pair#Reductive_group_over_a_local_field_with_closed_subgroup).


Here's why I think it should be true. Firstly, the analogous result is true if we replace the field extension $E$ with $F \oplus F$; this is the main result of the Harris--Scholl paper cited below. Secondly, it's true if $\pi$ is discrete-series: by Frobenius reciprocity we have $Hom\_{B(F)}(\pi, \mathbf{C}) = Hom\_{GL\_2(F)}(\pi, Ind\_{B(F)}^{GL\_2(F)}(\mathbf{C}))$. The representation $Ind\_{B(F)}^{GL\_2(F)}(\mathbf{C})$ has a trivial subrepresentation and the quotient is the Steinberg representation $\mathrm{St}$, so we have a left-exact seq
$$0 \to Hom\_{GL\_2(F)}(\pi, \mathbf{C}) \to Hom\_{B(F)}(\pi, \mathbf{C}) \to Hom\_{GL\_2(F)}(\pi, \mathrm{St}).$$
From the results in Prasad's 1992 paper cited below, exactly one of $Hom\_{GL\_2(F)}(\pi, \mathbf{C})$ and $Hom\_{GL\_2(F)}(\pi, \mathrm{St})$ is 1-dimensional and the other is zero, so we are done.


However, if $\pi$ is a principal series, we have the same exact sequence but it can happen that $Hom\_{GL\_2(F)}(\pi, \mathrm{St})$ and $Hom\_{GL\_2(F)}(\pi, \mathbf{C})$ are both non-zero (this occurs iff $\pi$ is the normalised induction of a pair of characters of $E^\times$ which are distinct, and both trivial on $F^\times$). I tried to bash out this case via Mackey theory, but I couldn't get it to work.


*Harris, M.; Scholl, A. J.*, **A note on trilinear forms for reducible representations and Beilinson’s conjectures.**, J. Eur. Math. Soc. (JEMS) 3, No. 1, 93-104 (2001). [Preprint version](https://www.dpmms.cam.ac.uk/%7Eajs1005/preprints/trilinear.pdf) [Published version](http://dx.doi.org/10.1007/s100970000026)


*Prasad, Dipendra*, [**Invariant forms for representations of $\text{GL}\_2$ over a local field**](http://dx.doi.org/10.2307/2374764), Am. J. Math. 114, No. 6, 1317-1363 (1992).



Branching laws for smooth representations
You can approach the problem via the mirabolic subgroup $P\_2(F)\subset B\_2(F)$. First we can restrict to $\pi$ with central character trivial on $F^\times$. Then you want to know if in this situation $Hom\_{P\_2(F)}(\pi,1)$ is of dimension at most $1$. This is indeed the case for unitary representations for example by [Matringe, Pacific Journal 2014, Proposition 2.5]. In particular this seems to take care of the remaining case you had of representations of the form $\chi\_1\times \chi\_2$ with $(\chi\_i)\_{|F^\times}=1$, which are clearly unitary as both $\chi\_i$'s have to be unitary in this case.


