[1] BORŮVKA O.: Lineaг Diffeгential Transformations of the Second Order. The English Univeгsities Press, London 1971.
[2] NEUMAN F.:
Constгuction of diffeгential equations with coexisting periodic solutions. Bull. Inst. Pol. Jassy (Romania) 16 (20), 1970, 67-75.
MR 0283305