This is known as the Binet equation. Integrating (1) yields the solution for ϕ φ φ = φ φ 0 + ∫ ∫ 1 r d u C − − u 2 − − G ( u ) where ϕ 0 is another constant of integration. A central-force problem is said to be "integrable" if this final integration can be solved in terms of known functions.