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5. It should be noted that the nonlinearity of the gravitational-field equations precludes the conclusion that gravitation is necessarily a weak force, since a perturbation expansion cannot always be made. Indeed,Efinger (6); has shown by solving the full nonlinear gravitational-field equationsexactly, that one can construct a stable charged particle wherein gravitational attraction balances electrical repulsion; however, if a perturbation expansion had been made, no solution would have been obtained. Other authors have also obtained a self-consistent solution (7).R. Arnowitt, S. Deser andC. W. Misner:The Dynamics of General Relativity, Chap. 5, p. 223 of the text:Gravitation, An Introduction to Current Research (New York, 1962). Relevant references are given here.