1. J.-P. Aubin, H. Frankowska, Set-Valued Analysis (Birkhäuser, Boston, 1990)
2. J.M. Borwein, Q.J. Zhu, A survey of subdifferential calculus with applications. Nonlinear Anal. Theory Methods Appl. 38(6), 687–773 (1999)
3. J.M. Borwein, Q.J. Zhu, Techniques of Variational Analysis. CMS Books in Mathematics/Ouvrages de Mathématiques de la SMC, vol. 20 (Springer, New York, 2005)
4. F.H. Clarke, Necessary Conditions for Nonsmooth Problems in Optimal Control and the Calculus of Variations. Ph.D. Dissertation, University of Seattle, Washington, 1973
5. F.H. Clarke, Generalized gradients and applications. Trans. Am. Math. Soc. 205, 247–262 (1975)