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2. Necessary conditions for nonsmooth variational problems;Clarke,1974
3. Generalized gradients and applications;Clarke;Trans. Amer. Math. Soc.,1975
4. Convex Analysis;Rockafellar,1970
5. Conjugate convex functions in optimal control and the calculus of variations;Rockafellar;J. Math. Anal. Appl.,1970