1. K. Schwarzschild, Sitz. Deut. Akad. Wiss. Berlin Kl. Math. Phys. 1916, 189 (1916). arXiv:physics/9905030
2. K. Schwarzschild, Sitz. Deut. Akad. Wiss. Berlin Kl. Math. Phys. 1916, 424 (1916). arXiv:physics/9912033
3. G. Lemaitre, Ann. Soc. Sci. Brux. A 53, 51 (1933)
4. R. Ruderman, Class. Ann. Rev. Astron. Astrophys. 10, 427 (1972)
5. R.L. Bowers, E.P.T. Liang, Astrophys. J. 188, 657 (1974)