1. Müller-Hartmann, E., Zittartz, J.: Phys. Rev. Letters33, 893 (1974)
2. Zittartz, J.: In: Proceedings of the International Symposium on Mathematical Problems in Theoretical Physics (ed. H. Araki), Kyoto, Japan (1975), to be published
3. Meanwhile it has been shown that the classicalX Y-model in two dimensions also exhibits the phase transition of continuous order: J. Zittartz, to be published
4. See, e.g.: Ruelle, D.: Statistical Mechanics, Amsterdam: W.A. Benjamin, Inc., 1969
5. Eggarter, T.P.: Phys. Rev. B9, 2989 (1974)