1. Kasteleyn, P.W.: J. Math. Phys.4, 1332 (1963)
2. McCoy, B., Wu, T.T.: The two-dimensional Ising Model. Cambridge, Mass.: Harvard University Press (1973)
3. Fisher, M.E.: J. Math. Phys.7, 1776 (1966)
4. The standard reference concerning the modular group is Klein, F., Fricke, R.: Vorlesungen über die Theorie der elliptischen Modulfunktionen. Leipzig: Teubner 1890. A concise summary of its main properties is given in Chapter I of Gunning, R.C.: Lectures on modular forms. Princeton: Princeton University Press 1962. Facts concerning fractional linear transformations, hyperbolic geometry and discontinuous groups may be found in Chapter 3 of Siegel, C.L.: Topics in complex function theory. London: Wiley 1971
5. See Magnus, W.: Non-euclidean tesselations and their groups. New York-London: Academic Press 1973