New THE MATHEMATICA JOURNAL article:
Manipulating Subgroups of the Modular Group
by DANIEL SCHULTZ
ABSTRACT: We describe efficient algorithms for working with subgroups of \text{PSL}_2 (\mathbb{Z}) . Operations discussed include join and meet, congruence testing, congruence closure, subgroup testing, cusp enumeration, supergroup lattice, generators and coset enumeration, and constructing a group from a list of generators.
