no. 3
The unramified principal series of p-adic groups. I. The spherical function
Compositio Mathematica, Tome 40 (1980) no. 3, pp. 387-406. 
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     author = {Casselman, William},
     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},
     zbl = {0472.22004},
     mrnumber = {571057},
     language = {en},
     url = {http://www.numdam.org/item/CM_1980__40_3_387_0/}
}
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Casselman, W. The unramified principal series of $p$-adic groups. I. The spherical function. Compositio Mathematica, Tome 40 (1980) no. 3, pp. 387-406. http://www.numdam.org/item/CM_1980__40_3_387_0/

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