Degree 1 and 2 representations of nilpotent groups and applications to units of group rings. E. Jespers, G. Leal (1995) Manuscripta mathematica
Die harmonischen Involutionen der Permutationsgruppe P...L2 (K). Helmut Mäurer (1983) Mathematische Zeitschrift
Divisibility properties of subgroup numbers for the modular group. Müller, T.W., Schlage-Puchta, J.-C. (2005) The New York Journal of Mathematics [electronic only]
Doubles of groups and hyperbolic LERF 3-manifolds. Gitik, Rita (1999) Annals of Mathematics. Second Series