1. The support functionals of a convex set;Bishop,1963
2. Surles problemes variationnels;Ekeland;C. R. Acad. Sci. Paris,1972
3. On the variational principle;Ekeland;J. Math. Anal. Appl.,1974
4. Nonconvex minimization problems;Ekeland;Bull. Amer. Math. Soc.,1979
5. On a lemma of Bishop and Phelps;Brøndsted;Pacific J. Math.,1974