[1] B. P. Пemyxoв: Oб oднoм клacce диффspeнциaльнo-фyнкциoнaльныx ypaвнeний. Tp. ceминapa пo тeopии диф. ypaвн. c oткл. apг. 4 (1967), 139-148.
[2] J. A. Yorke:
Tһe period of periodic solutions and charged particles in magnetic fields. Lecture Notes in Mathematics 144 (1970), 267-268.
MR 0385246