1. PaisA. and UhlenbeckG. E.,Phys. Rev. 76, 145 (1950).
2. StelleK. S.,Phys. Rev. D16, 953 (1977);Gen. Rel. Grav. 9, 353 (1978).
3. BoulwareD. G., DeserS., and StelleK. S., in I. A.Batalin et al. (eds),QFT and Quantum Statistics, Vol. 2, Adam Hilger, Bristol, 1987.
4. HawkingS. W., in I. A.Batalin et al. (eds),QFT and Quantum Statistics, Vol. 2, Adam Hilger, Bristol, 1987.
5. LeeT. D. and WickG. C.,Nucl. Phys. B9, 209 (1969).