Stationary two-black-hole configurations: A non-existence proof
  Based on the solution of a boundary problem for disconnected (Killing) horizons and the resulting violation of characteristic black hole properties, we present a non-existence proof for equilibrium configurations consisting of two aligned rotating black holes. Our discussion is principally aimed at developing the ideas of the proof and summarizing the results of two preceding papers (Neugebauer and Hennig, 2009 [29], Hennig and Neugebauer, 2011 [12]). From a mathematical point of view, this paper is a further example (Meinel et al., 2008 [22]) for the application of the inverse ("scattering") method to a non-linear elliptic differential equation. 
Gernot Neugebauer and J\"org Hennig
gr-qc