Let $G$ be a simple compact connected Lie group and let $K$ be a connected closed subgroup of $G$. Let $\widehat G$ and $\widehat K$ denote the corresponding unitary duals, that is, the (equivalence classes of) irreducible representations. By the Highest Weight Theorem, elements in $\widehat G$ are in correspondence with elements in the set $P^+(G)$ of $G$-integral dominant weights. For $\lambda\in P^+(G)$, we will denote by $\pi_\lambda$ the irreducible representation of $G$ with highest weight $\lambda$.

Given $\tau\in\widehat K$, we define $\widehat G_\tau=\{\pi\in\widehat G: \textrm{Hom}_K(\pi,\tau)\neq0\}$. In other words, $\pi$ is in $\widehat G_\tau$ if and only if $\tau$ appears in the decomposition in irreducible representations of the restriction of $\pi|_K$ (the restriction of $\pi$ to $K$). Elements in $\widehat G_\tau$ are called $\tau$-spherical representations.

I wish to know if there is $\omega\in P^+(G)$, with $\omega\neq0$, satisfying the following property:

if $\lambda\in P^+(G)$ satisfies $\pi_\lambda\in\widehat G_\tau$, then $\pi_{\lambda+\omega}\in\widehat G_\tau$.

Here I list some examples coming from compact symmetric spaces of real rank one. In each case I used the obvious inclusion $K\subset G$ and standard conventions on the root system of $\mathfrak g$:

- $G$ arbitrary and $K=\{1\}$, $\omega =$ any choice in $P^+(G)$.
- $G=SU(2)$ and $K=T$ (maximal torus), $\omega = 2\varepsilon_1$ (twice the first fundamental weight).
- $G=SO(n+1)$ and $K=SO(n)$, $\omega=\varepsilon_1$ (the first fundamental weight).
- $G=SU(n+1)$ and $K=S(U(n)\times U(1))$, $\omega=\varepsilon_1-\varepsilon_{n+1}$ (the first plus the last fundamental weights).
- $G=Sp(n+1)$ and $K=Sp(n)$, $\omega=\varepsilon_1+\varepsilon_2$ (the second fundamental weight).
- $G=SO(2n)$ and $K=T$ (max. torus), $\omega= 2\varepsilon_1$ (twice the first fundamental weight).

Each of the above examples follows by the explicit branching law from $G$ to $K$, which is (of course) not available for arbitrary $K$. Clearly, in each case, any positive multiple of the indicated $\omega$ also works.

In case the answer is affirmative, it follows that $\widehat G_\tau$ can always be written as a union of subsets of the form $\{\pi_{\lambda+k\omega}: k\geq0 \}$ for some $\lambda\in P^+(G)$. (The union is not assumed finite or either disjoint.) This is the result I am looking for.

**Edit:** Looking for more information, here is an intermediate question:

For $G=SO(2n)$. Does a positive multiple of $\varepsilon_1$ satisfy the above property for any closed subgroup $K$ of $G$?

Either a proof or a counterexample would help me to understand this problem. I picked $SO(2n)$ because is my favorite, but we may replace $(SO(2n),\varepsilon_1)$ in the question by $(SO(2n+1),\varepsilon_1)$, $(SU(n+1),\varepsilon_1-\varepsilon_{n+1})$, or $(Sp(n+1),\varepsilon_1+\varepsilon_2)$.

anyrepresentation of $G$ is spherical, so the property trivially holds for any $\omega$. Also, you should exclude the dominant weight $\omega=0$, again, the property holds trivially. $\endgroup$ – Victor Protsak Apr 11 '19 at 5:19