%0 Journal Article %A Casselman, W. %T The unramified principal series of $p$-adic groups. I. The spherical function %J Compositio Mathematica %D 1980 %P 387-406 %V 40 %N 3 %I Sijthoff et Noordhoff International Publishers %U http://www.numdam.org/item/CM_1980__40_3_387_0/ %G en %F CM_1980__40_3_387_0