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Published online by Cambridge University Press: 24 March 2025
Without using the $p$-adic Langlands correspondence, we prove that for many finite-length smooth representations of
$\mathrm {GL}_2(\mathbf {Q}_p)$ on
$p$-torsion modules the
$\mathrm {GL}_2(\mathbf {Q}_p)$-linear morphisms coincide with the morphisms that are linear for the normalizer of a parahoric subgroup. We identify this subgroup to be the Iwahori subgroup in the supersingular case, and
$\mathrm {GL}_2(\mathbf {Z}_p)$ in the principal series case. As an application, we relate the action of parahoric subgroups to the action of the inertia group of
$\mathrm {Gal}(\overline {\mathbf {Q}}_p/\mathbf {Q}_p)$, and we prove that if an irreducible Banach space representation
$\Pi$ of
$\mathrm {GL}_2(\mathbf {Q}_p)$ has infinite
$\mathrm {GL}_2(\mathbf {Z}_p)$-length, then a twist of
$\Pi$ has locally algebraic vectors. This answers a question of Dospinescu. We make the simplifying assumption that
$p > 3$ and that all our representations are generic.
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