1. F Károlyházy, Nuovo Cimento A42, 390 (1966)
2. A B Komar, Int. J. Theor. Phys. 2, 157 (1969)
3. T W B Kibble, in Quantum gravity 2: A second oxford symposium edited by C J Isham, R Penrose and D W Sciama (Oxford University Press, Oxford, 1981) p. 63
4. R Penrose, in Quantum gravity 2: a second oxford symposium edited by C J Isham, R Penrose and D W Sciama (Oxford University Press, Oxford, 1981) p. 224
5. L Diosi, Phys. Lett. A120, 199 (1984)