Toshikazu Takagi. Modified Siegel functions relative to the principal congruence subgroups of $G(\sqrt{M}). Journal of the Faculty of Arts and Sciences at Fujiyoshida, Showa University. 2009. 4. 1-16
Toshikazu Takagi. Classification of the principal congruence subgroups of the group $G(\sqrt{M}). Journal of the Faculty of Arts and Sciences at Fujiyoshida, Showa University. 2007. 2. 1-20