1. Kosterlitz, J.M., Thouless, D.J.: J. Phys. C6, 1181 (1973)
2. Kosterlitz, J.M., Thouless, D.J.: Progress in Low Temperature Physics. Brewer, D.F. (ed.), Vol. VII B. Amsterdam, Oxford, New York: North-Holland 1978
3. See, e.g., Nelson, D.R.: In: Phase transitions and critical phenomena. Vol. 7. Domb, C., Lebowitz, J. (eds.), Vol. 7. New York: Academic Press 1983
4. See, e.g., Nelson, D.R., Kosterlitz, J.M.: Phys. Rev. Lett.39, 1201 (1977)
5. See, e.g., Regnault, L.P., Rosat-Mignod, J.: In: Magnetic properties of layered transition metal compounds. Jongh, L.J. de, Willett, R.D. (eds.) (in press)