Andreas de Vries, Reinhold Böhme & Theodor Schmidt-Kaler, published in: Astrophysics and Space Sciences 225, 221-226 (1995), doi:10.1007/BF00613236
On one part this paper bases on the fundamental approach to apply linear functional analytic methods in general relativity theory and to consider the Maxwell field equations around a black hole as evolution equations.
Combining this result with the COBE observation of anisotropies in the cosmic microwave background radiation, the total mass M of all black holes in the observable universe is bounded by
M < 2.4 · 1085 mp = 4 · 1058kg,
where mp denotes the proton mass, mp = 1.6·10-27kg.
This is the only paper I published in cooperation with my admired academic teacher R. Böhme. He died on May 21, 2000.