Guggenheimer, H. Hill equations with coexisting periodic solutions. (English) Zbl 0167.37003 J. Differ. Equations 5, 159-166 (1969). PDFBibTeX XMLCite \textit{H. Guggenheimer}, J. Differ. Equations 5, 159--166 (1969; Zbl 0167.37003) Full Text: DOI
Heil, E. Abschätzungen für einige Affininvarianten konvexer Kurven. (German) Zbl 0153.50902 Monatsh. Math. 71, 405-423 (1967). PDFBibTeX XMLCite \textit{E. Heil}, Monatsh. Math. 71, 405--423 (1967; Zbl 0153.50902) Full Text: DOI EuDML