Representations of $GL_n(D)$ near the identity
For a central division algebra $D$ of dimension $d^2$ over a finite extension $F$ of $\mathbb Q_p$ or of $\mathbb F_p((t))$, a field $R$ of characteristic prime to $p$, and an irreducible smooth $R$-representation $π$ of $G=GL_n(D)$, we show that for small enough compact open pro-$p$ subgroup $K$ of $G$, the restriction of $π$ to $K$ is the same as that of a virtual representation $\sum c_π(λ) Ind_{P_λ}^G 1$, where the sum is over partitions $λ$ of $n$ and $P_λ$ a parabolic subgroup of $G$ associated to $λ$. When $K$ is a Moy-Prasad subgroup of $G$ we determine from the $c_π(λ)$ a polynomial $P_{π,K}$ of degree $d(π)$ independent of the choice of $K$, such that for large enough integers $j$ the dimension of the points of $π$ fixed under the congruence subgroup $K_j$ of $K$ is $P_{π,K}(q^j)$ where $q$ is the cardinality of the residue field of $D$.