Finite difference methods for computing eigenvalues of fourth order boundary value problems. Usmani, Riaz A. (1986) International Journal of Mathematics and Mathematical Sciences
Floquet operators with singular spectrum. I James S. Howland (1989) Annales de l'I.H.P. Physique théorique
Floquet operators with singular spectrum. II James S. Howland (1989) Annales de l'I.H.P. Physique théorique