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The n-body problem is the problem of predicting the motion of a group of celestial objects that interact with each other gravitationally. Solving this problem has been motivated by the need to understand the motion of the sun, planets and the visible stars. Its first complete mathematical formulation appeared in Isaac Newton's Principia (the n-body problem in general relativity is considerably more difficult). Since gravity was responsible for the motion of planets and stars, Newton had to express gravitational interactions in terms of differential equations. Newton proved in the ...