@article{CM_1980__40_3_387_0,
     author = {Casselman, W.},
     title = {The unramified principal series of $p$-adic groups. {I.} {The} spherical function},
     journal = {Compositio Mathematica},
     pages = {387--406},
     publisher = {Sijthoff et Noordhoff International Publishers},
     volume = {40},
     number = {3},
     year = {1980},
     mrnumber = {571057},
     zbl = {0472.22004},
     language = {en},
     url = {http://www.numdam.org/item/CM_1980__40_3_387_0/}
}