1. Amann, H., Ambrosetti, A., Mancini, G.: Elliptic equations with noninvertible Fredholm linear part and bounded nonlinearities. Math. Z. 158, 179–194 (1978)
2. Barbu, V.: Abstract periodic Hamiltonian systems. Adv. Differ. Equ. 1, 675–688 (1996)
3. Bebernes, J.W., Schmitt, K.: Periodic boundary value problems for systems of second order differential equations. J. Differ. Equ. 13, 32–47 (1973)
4. Boscaggin, A., Fonda, A., Garrione, M.: An infinite-dimensional version of the Poincaré–Birkhoff theorem. Ann. Scuola Norm. Pisa 20, 751–770 (2020)
5. De Coster, C., Habets, P.: Two-Point Boundary Value Problems. Lower and Upper Solutions. Elsevier, Amsterdam (2006)