We present a theoretical view of computation, delineating what types of approximations are possible and what types are impossible. A practical consequence is an approximation algorithm with numerous applications. For several classical problems (finding maximizers, fixed points, equilibrium...

We prove that the Strong Axiom of Revealed Preference tests the existence of a strictly quasiconcave (in fact, continuous, generically C(infinity), strictly concave, and strictly monotone) utility function generating finitely many demand observations. This sharpens earlier results of Afriat,...