On eternal black holes and some interior solutions
  In this article we deduce two new exact solutions of Einstein's equations for eternal black holes, now related to stiff matter, one `static' and another rotating (stationary like the Kerr one), thus the number of these eternal solutions grows from four to six. Dealing with these static and rotating solutions we automatically rehabilitate the practically forgotten Sommerfeld-Lenz approach to the deduction of our results for the satisfaction of Einstein's equations, e.g., in these rotating cases. At the same time, we come to the confirmation of our new relativistic generalization of the inertial/gravitational mass equivalence principle, as well as to obtention of the Kerr interior solution. 
Nikolai V. Mitskievich
gr-qc