1. Proximity maps for convex sets;Cheney, Ward;Proc. Amer. Math. Soc.,1959
2. An operator theoretic formulation of a class of control problems and a steepest descent method of solution;Balakrishnan, A. V.;J. SIAM Control Ser. A,1963
3. Minimizing functionals on Hilbert space;Goldstein, A. A.,1964