1. W.Betori - M.Pagliacci,Harmonic analysis for groups acting on tree, preprints 1982.
2. P. Cartier,Harmonic analysis on trees, Proc. Symp. Pure Math. Amer. Math. Soc.,26 (1972), pp. 419–424.
3. J. Cohen,Operators norms on free groups, Boll. Un. Mat. Ital., (VI)1-B (1982), pp. 1055–1065.
4. A. Figà-Talamanca -M. Picardello,Spherical functions and harmonic analysis on free groups, J. Funct. Analysis,47 (1982), pp. 281–304.
5. A.Figà-Talamanca - M.Picardello,Harmonic analysis on free groups, to appear in Lecture Notes in Pure and Applied Mathematics, Marcel Dekker.