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2. Sarkissian, B.V.B., Coles, B.R.: Comm. on Phys. 1 (1976) 17.
3. Sarkissian, B.V.B.: J. Phys. F 7 (1977) L 139.
4. Dumas, J., Schlenker, C., Tholence, J.L., Tournier, R.: Phys. Rev. B 20 (1979) 3913.
5. Mikayako, Y., Kimishima, Y., Sato, K.: unpublished data.