Periodic solutions of nearly integrable Hamiltonian systems bifurcating from infinite-dimensional tori

Periodic Solutions of Second-Order Differential Equations in Hilbert Spaces

“…The study of periodic solutions for infinite-dimensional Hamiltonian systems has been already faced by several authors, see, e.g., [2,4,8,9,11]. Our approach does not need a Hamiltonian structure and could be applied also to systems with nonlinearity depending on the derivative of x, provided some Nagumo-type condition is assumed.…”