1. Y. Imry: In Chemistry and Physics of Solid Surfaces II, ed. By R. Vanselow (CRC, Boca Raton 1979) p.461; J.M. Kosterlitz: In Phase Transitions in Surface Films, ed. By G. Dash, J. Ruvalds ( Plenum, New York 1980 ) p. 193
2. B.I. Halperin, D.R. Nelson: Phys. Rev. Lett. 41, 121, E515 (1978); D.R. Nelson, B.I. Halperin: Phys. Rev. B19, 2457 (1979)
3. L. Onsager: Phys. Rev. 65, 117 (1944)
4. a R.J. Baxter: J. Phys. A13, L61 (1980)
5. b R.J. Baxter, F.Y. Wu: Phys. Rev. Lett. 31, 1294 (1973); Aust. J. Phys. 27, 357 (1974) R.J. Baxter: Phys. Rev. Lett. 26, 832 (1971); Ann. Phys. N.Y. 70, 193 (1972)