1. V. Benci, A. Capozzi, and D. Fortunato, Periodic solutions of Hamiltonian systems with superquadratic potential. Ann.Mat.Pura Appl., Ser.4 143 (1986), p. 1–46
2. M.S.P. Eastham, “The Spectral Theory of Periodic Differential Equations”. Scottish Academic Press, Edinburgh 1973
3. H.P. Heinz, Bifurcation from the essential spectrum for nonlinear perturbations of Hill’s equation, in: “Differential Equations—Stability and Control” (ed. S. Elaydi ), Marcel Dekker, New York 1990, pp. 219–226
4. H.P. Heinz, Lacunary bifurcation for operator equations and nonlinear boundary value problems on RN, preprint
5. H.P. Heinz and C.A. Stuart, Solvability of nonlinear equations in spectral gaps of the linearization, preprint