The unramified principal series of p -adic groups. I. The spherical function W. Casselman — 1980 Compositio Mathematica
On Abelian Varieties with Many Endomorphisms and a Conjecture of Shimura's. W. Casselman — 1971 Inventiones mathematicae
The unramified principal series of p -adic groups. II. The Whittaker function W. Casselman; J. Shalika — 1980 Compositio Mathematica