Free subgroups in certain generalized triangle groups of type (2, m, 2)

A generalized triangle group is a group that can be presented in the form G = p = y^q = w(x,y)^r = 1> where p,q,r = 2 and w(x,y) is a cyclically reduced word of length at least 2 in the free product Z_p * Z_q = p = y^q = 1>. Rosenberger has conjectured that every generalized triangle group G satisfies the Tits alternative. It is known that the conjecture holds except possibly when the triple (p,q,r) is one of (3, 3, 2), (3, 4, 2), (3, 5, 2), or (2, m, 2) where m=3, 4, 5, 6, 10, 12, 15 , 20, 30, 60. In this paper, we show that the Tits alternative holds in the cases (p,q,r)=(2, m, 2) where m=6, 10, 12, 15, 20, 30, 60.