1. Avis, S. J., and Isham, C. (1979). Generalised spin structures on four-dimensional space-times, ICTP/78-79/21.
2. Bugajska, K. (1985).Journal of Mathematical Physics,26, 2909?2914.
3. Bugajska, K. (1986).Journal of Mathematical Physics,27, 143?150.
4. Bugajska, K. (1987a). Lorentz structures of open spin manifolds, submitted for publication.
5. Bugajska, K. (1987b). Internal structure of space-time, inJourn�es Relativistes de Toulouse 1986, Springer-Verlag.