1. Thompson, C.J., Kac, M.: Phase transitions and eigenvalue degeneracy of a one-dimensional anharmonic oscillator. Studies Appl. Math.48, 257–264 (1969)
2. Isaacson, D.: Singular perturbations and asymptotic eigenvalue degeneracy. Commun. Pure Appl. Math.29, 531–551 (1976)
3. Reed, M., Simon, B.: Methods of modern mathematical physics, Vol. 4. New York: Academic Press 1978
4. Kato, T.: Perturbation theory for linear operators. Die Grundlehren der mathematischen Wissenschaften, Vol. 132. Berlin-Heidelberg-New York: Springer-Verlag 1966
5. Reed, M., Simon, B.: Methods of modern mathematical physics, Vol. III. New York: Academic Press 1978