1. Andronov, A.A., Witt, A.A., Haikin, S.E.: Oscillation Theory. Physmatgiz, Moscow (1959)
2. Choquet-Bruhat, Y., DeWitt-Morette, C.: Analysis, Manifolds and Physics, Part 1: Basics. Elsevier Science B.V., North Holland (1982)
3. Kriuchkov, V.N., Solomonovich, M.: Self-sustained oscillation in economics. Appl. Math. Comput. 49, 63–77 (1992)