On a problem of Fujii concerning Riemann's ζ -function. Puchta, J.-C. — 2001 Acta Mathematica Universitatis Comenianae. New Series
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]