Classify all representation which contains a Steinberg in its hyperspecial subgroup

Abstract: This paper addresses Question 1 posed by Dipendra Prasad in his recent problem list: classify all irreducible smooth representations of an unramified reductive p-adic group such that the space of vectors fixed by the pro-unipotent radical of a hyperspecial maximal compact subgroup, viewed as a representation of the finite reductive group obtained as the quotient of that hyperspecial subgroup by its pro-unipotent radical, contains the Steinberg representation. We prove that any such representation must be Iwahori-spherical, hence a subquotient of some unramified principal series. By a detailed analysis of the action of the Iwahori--Hecke algebra on the Iwahori-fixed space, we show that in each principal series there exists exactly one irreducible subquotient containing the Steinberg representation, and we give an explicit construction of this subquotient. Hence this gives a bijection between Weyl group orbits of unramified characters and irreducible representations containing the Steinberg representation in their hyperspecial subgroup. This classification is richer than Prasad's expectation: besides twists of the Steinberg representation and generic unramified representations, every principal series contributes a unique subquotient with the required property. The proof relies on establishing an isomorphism between the finite Iwahori--Hecke algebra of the finite Lie group obtained as the quotient of the hyperspecial subgroup by its pro-unipotent radical and the finite Hecke algebra of the p-adic group consisting of compactly supported I-bi-invariant functions on the hyperspecial subgroup, and comparing their actions on the Iwahori-fixed vectors.