1. H Brown, R Bülow, J Neubilser, H Wondratschek, H Zassenhaus 1978, Crystallographic Groups of Four-Dimensional Space,Wiley, New York
2. M Baake, D Joseph and P Kramer 1992, Phys Lett A168 199–208
3. F Klein and R Fricke 1890–92, Vorlesungen iiber die Theorie der Elliptischen Modulfunktionen, vol.1, pp. 218–219, 452–455
4. A G Kurosh 1960, The Theory of Groups, vol. 2, pp. 261–264 Chelsea, New York
5. A Janner, E Ascher 1969, Physica 45 33–66