1. R. T. Rockafellar. The theory of subgradients and its applications to problems of optimization. Lecture Notes Series, Montreal Univ. Press, Montreal, 1978.
2. F. H. Clarke. Optimization and nonsmooth analysis. John Wiley & Sons Publ. Co., New York, 1983.
3. J. Warga. Optimal control of differential and functional equations. Academic Press, New York, 1972.
4. J.-P. Penot. Calcul sous-différentiel et optimization. J. Funct. Anal. 27, 248–276, 1978.
5. A. Y. Kruger, B. S. Mordukhovich. Extremal points–the Euler equation in nonsmooth optimization problems. Dokl. Acad. Nauk BSSR 24, 684–687, 1978.