1. H. Guggenheimer,Hill Equations with coexisting periodic solutions II, Comment. Math Helv., to appear.
2. E. P. Lane,A Treatise on Projective Differential Geometry, Chicago, 1942.
3. W. Magnus and S. Winkler,Hill’s Equation, New York, 1966.
4. E. J. Wilczynski,Projective Differential Geometry of Curves and Ruled Surfaces, Leipzig, 1905, New York.