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2. Angell, T. S., Existence theorems for a class of optimal control problems with delay, Ph.D. thesis, University of Michigan, 1969.
3. Existence theorems for optimal control problems involving functional differential equations;Angell;J. Optimization Theory and Applications,1971
4. Filippov existence theorem for optimal control of a class of hereditary systems;Angell,1970
5. The optimization of trajectories of linear functional differential equations;Banks;SIAM J. Control,1970