1. Dz. H. Karimov, On periodic solutions of nonlinear differential equations of parabolic type. Doklady Akad. Nauk SSSR (N.S.) 58, 969–972 (1947). [Russian].
2. R. Lemmert, unpublished (the results are part of Mr. Lemmert's doctoral dissertation which will soon be published).
3. Oleinik, O.A., On the system of equations of boundary layer theory. Z. Vycisl. Mat. i Mat. Fiz. 3, 489–507 (1963).
4. Oleinik, O.A., The Prandtl system of equations in boundary layer theory. Dokl. Akad. Nauk SSSR 150 — Soviet Math. 4, 583–586 (1963).
5. G. Prodi, Soluzioni periodiche di equazioni alle derivate parziali di tipo parabolico e non lineari. Rivista Mat. Univ. Parma 3, 265–290 (1952).