1. G.L. Lamb, Jr., from the letter to the author.
2. L.P. Eisenhart: A Treatise on the Differential Geometry of Curves and Surfaces (New York, N.Y., 1960)
3. L. Bianchi: Lezioni di geometria differenziale (Pisa, 1922)
4. R.L. Anderson and N.H. Ibragimov: Lie-Bäcklund transformations in applications(SIAM, Philadelphia 1979)
5. E.L. Ince: Ordinary Differential Equations(Dover, N.Y. 1956)