Addenda to “Periodic solutions of x " + f ( x ) x ' 2 n + g ( x ) = 0 with arbitrarily large period” J. W. Heidel — 1973 Annales Polonici Mathematici
Periodic solutions of x " + f ( x ) x ' 2 n + g ( x ) = 0 with arbitrarily large period J. W. Heidel — 1971 Annales Polonici Mathematici