1. M. Morse, The Calculus of Variations in the Large, Colloquium Publications, American Mathematical Society, Providence, 18 (1934).
2. G. Reeb, “Sur les points singuliers d’une forme de Pfaff complètement intégrable ou d’une fonction numérique,” C.R.A.S., 222, 847–849 (1946).
3. A. S. Kronrod, “On functions of two variables,” Usp. Mat. Nauk, 5, No. 1(35), 24–134 (1950).
4. A. O. Prishlyak, Topological Properties of Functions on Two and Three Dimensional Manifolds, Palmarium Acad. Publ., Saarbrücken (2012).
5. V. V. Sharko, Functions on Manifolds. Algebraic and Topological Aspects, American Mathematical Society, Providence, RI (1993).