Numerical verification of existence and inclusion of solutions for nonlinear operator equations

Nonlinear operator equations of the type f(u) ≡ Lu + Nu = 0, u ∈ D(L) are considered, where L is a closed linear operator from a Banach space X to another Banach space Y and N a nonlinear operator from X to Y. A method is presented for numerical verification and inclusion of solutions for the equations. As an example, the existence of a periodic solution is proved for the Duffing equation.