1. Collatz, L., Eigenwertaufgaben mit technishen
Anwendungen, Leipzig: Geest & Portig K.-G., 1963.
2. Van’ko, V.I., On eigenfrequencies of vibrations of wires on overhead power
transmission lines, Izv. VUZov, 1987, no. 8,
pp. 48–56.
3. Brezis, H. and Nirenberg, L., Characterizations of the ranges of some nonlinear
operators and applications to boundary value problems, Ann. Scuola
Norm. Sup. Pisa, 1978, vol. 5, pp. 225–325.
4. Rabinowitz, P., Free vibration for a semilinear wave equation, Comm. Pure Appl. Math., 1978, vol. 31, no. 1,
pp. 31–68.
5. Tanaka, K., Infinitely many periodic solutions for the equation:
$$
u_{tt}-u_{xx}\pm |u|^{p-1}u=f(x,t).$$ II,Trans. Am. Math. Soc., 1988, vol. 307,
pp. 615–645.