1. V.M. Alekseev, V.M. Tikhomizov and S.V. Fomin, Optimal control (in Russian) (Nauka, Moscow, 1979).
2. F.H. Clarke, “The maximum principle under minimal hypothesis”, SIAM Journal on Control and Optimization 14(6). (1976) 1078–1091.
3. V.F. Demyanov and A.M. Rubinov, “On quasidifferentiable functionals”, Doklady Akademii Nauk SSSR 250 (1980) 21–25. (Translated in Soviet Mathematics Doklady 21(1) (1980) 14–17.)
4. V.F. Demyanov and A.M. Rubinov, “On quasidifferentiable mappings”, Mathematische Operationsforschung und Statistik, Series Optimization 14(1) (1983) 3–21.
5. V.F. Demyanov, ed., Nonsmooth problems of optimization theory and of control (in Russian) (Leningrad University Press, Leningrad, 1982) (see especially chapter IV by V.N. Nikulina and I.R. Shablinskaya).