1. Adler S L 1982Rev. Mod. Phys. 54 729
2. Chela-Flores J and Varela V 1983Phys. Rev. D27 1248
3. Dennis P W and Huang J C 1977Phys. Rev. D15 983
4. Duff M F, Nilsson B E W and Pope C N 1983 inRelativity cosmology, topological mass and supergravity (ed.) C Aragone, (Singapore: World Scientific)
5. Eguchi T, Gilkay P B and Hanson A J 1980Phys. Rep. 66 213