[1] O. Vejvoda: The mixed problem and periodic solutions for a linear and weakly nonlinear wave equation in one dimension. Rozpravy CSAV, řada matematických a přírodnich věd, 1970, rocnik 80, sesit 3, Academia Praha. MR 0310437 | Zbl 0278.35064

[2] O. Vejvoda: Periodic solutions of a linear and weakly nonlinear wave equation in one dimension, I. Czech. Math. J., I4 (89), 1964, 341-382. MR 0174872

[3] N. Krylovd O. Vejvoda: A linear and weakly nonlinear equation of a beam: the boundary-value problem for free extremities and its periodic solutions. Czech. Math. J., 2I (96), 1971, 535 to 566. MR 0289918

[4] A. J. Hinčin: Continued fractions. (Russian), 3rd edition, Fizmatgiz, Moscow, 1961.