1. N. E. Firsova, ?The Riemann surface of a quasiimpulse, and scattering theory for a perturbed Hill operator,? J. Sov. Math.,11, No. 3 (1979).
2. E. C. Titchmarsh, Eigenfunction Expansions Associated with Second-Order Differential Equations, Part II, Oxford Univ. Press, Oxford (1958).
3. N. E. Firsova, ?A trace formula for a perturbed one-dimensional Schrödinger operator with a periodic potential. I,? Prob. Mat. Fiz., No. 7, 162?177 (1974).
4. V. A. Zheludev, ?On the perturbations of the spectrum of a one-dimensional Schrödinger operator with a periodic potential,? Candidate's Dissertation, Leningrad (1968).