1. Aomoto, K. (1984). Spectral theory on a free group and algebraic curves.J. Fac. Sci. Tokyo Sec. IA 31, 297?317.
2. Cartier, P. (1972). Fohctions harmoniques sur un arbre.Symp. Math. 9, 203?270.
3. Cartwright, D., and Soardi, P. (1986). Random walks on free products, quotients, and amalgams.Nagoya Math. J. 102, 163?180.
4. Derriennic, Y. (1975). Marches aleatoire sur le group libre et frontière de Martin.Z. Warsch. Verw. Gebiete 32, 261?276.
5. Doob, J., Snell, J. L., and Williamson, R. E. (1960). Applications of boundary theory to sums of independent random variables. In I. Olkin (ed.),Contributions to Probability and Statistics, Stanford University Press, Stanford, CA.