Congruence subgroups of groups commensurable with PSL ( 2 , ℤ ) of genus 0 and 1. Cummins, C.J. — 2004 Experimental Mathematics
Congruence subgroups of PSL ( 2 , ℤ ) of genus less than or equal to 24. Cummins, C.J.; Pauli, S. — 2003 Experimental Mathematics