Let $G$ be a reductive p-adic group, $\pi$ a complex smooth representation of $G$. Then it is known that if $\pi$ is irreducible, then it is admissible.
I need help to find a reference for this fact, and want to know if it is true for real reductive group. Thanks a lot.
1) In these lecture notes:
you have theorem 12, on page 37. He does it for GL_n I think, but it should be similar for other groups...
2) For a real group, I think you should be more specific. There is Soergel's counter-example in the paper "An Irreducible not Admissible Banach Representation of $SL(2, \mathbb{R})$".
Historically, the result is due to Hervé Jacquet
MR0369624 (51 #5856) Jacquet, Hervé Sur les représentations des groupes réductifs p-adiques. C. R. Acad. Sci. Paris Sér. A-B 280 (1975), Aii, A1271–A1272. 22E50
(From MathSciNet)
"Let F denote a nonarchimedean nondiscrete locally compact field and G a reductive F-group. A representation r or G(F) in a complex vector space V is said to be "smooth'' if the stabilizer of each vector in V is open in G(F); r is said to be "admissible'' if it is smooth and the space of vectors in V fixed by any open compact subgroup of G(F) is finite-dimensional. The author proves that every irreducible smooth representation of G(F) is automatically admissible. The proof, like the result itself, is surprisingly simple and clever. "
