CZECHOSLOVAK MATHEMATICAL JOURNAL, cilt.54, sa.1, ss.253-259, 2004 (SCI-Expanded)
Hecke groups H(lambda(q)) are the discrete subgroups of PSL(2, R) generated by S(z) = -(z + lambda(q))(-1) and T(z) = -1/z. The commutator subgroup of H(lambda(q)), denoted by H'(lambda(q)), is studied in [2]. It was shown that H'(lambda(q)) is a free group of rank q - 1.