[1] Borůvka O.: Linear Differentialtransformationen 2. Ordnung. VEB, Berlin 1967.
[2] Neuman F.:
Extremal Property of the Equation $y" =-k^2 y$. Archivum Mathematicum (Brno), T. 8 (1907), 161-164.
MR 0236465
[3] Neuman F.:
Construction of Differential Equations with Coexisting Periodic Solutions. to appear in Bul. Inst. Polit. din Jaşi.
MR 0283305 |
Zbl 0267.34039