1. J. Eells, Singularities of Smooth Maps, Thomas Nelson and Sons, 1968.
2. On the Double Tangents of Plane Closed Curves.
3. M. do Carmo, Differential Geometry of Curves and Surfaces, Prentice-Hall, 1976.
4. C. F. Gauss, Werke, Teubner, Leipzig, 1900, 282–286.
5. An inequality for double tangents