1. [A-L] Andronov A; Leontovich E. et al.,Theory of Bifurcations of Dynamical Systems on the plane, John Wiley, New York, 1973.
2. [Ch] Chuan-Chih Hsiung,A first course in differential geometry, John Wiley and Sons, N. Y. 1981.
3. [G-S.1] Gutierrez C., Sotomayor J.Structurally stable configurations of lines of principal curvature. Asterisque 98–99 (1982).
4. [G-S.2] Gutierrez C., Sotomayor J.An approximation theorem for immersions with stable configurations of lines of principal curvature. Springer Lectures Notes in Math. 1007, (1983).
5. [G-S.3] Gutierrez C., Sotomayor J.,Stability and bifurcations of configurations of principal lines. Preprint IMPA, Aport. Mat., 1 (1985), Soc. Mat. Mex.