1. J.M. Kosterlitz and D.J. Thouless: in Progress in Low-Temperature Physics, ed. by D.F. Brewer, Vol VII B ( North-Holland, Amsterdam 1978 ) p. 371
2. R. Peierls: Helv. Phys. Acta 7 (Suppl. II), 81 (1934)
3. L.D. Landau: Zh. Eksp. Teor. Fiz. 7, 627 (1937)
4. N. Mermin and H. Wagner: Phys. Rev. Lett. 17, 1133 (1966)
5. P.C. Hohenberg: Phys. Rev. 158, 383 (1967)