1. Courant, R., & D. Hilbert, Methods of Mathematical Physics. Vol. I, New York: Interscience Pub. Co. 1953.
2. Dunford, N., & J. T. Schwartz, Linear Operators. Part II. Spectral Theory. New York: Interscience Pub. Co. 1963.
3. Egorov, Yu. V., Certain problems in the theory of optimal control. Dokl. Akad. Nauk. SSSR 145, 241–244 (1962). (Russian; English translation in Soviet Mathematics 3, 1080–1084 (1962).)
4. Egorov, Yu. V., Some problems in the theory of optimal control. Ž. Vyčisl. Mat. Fiz. 5, 887–904 (1963).
5. Fattorini, H. O., Boundary control systems. SIAM Journal on Control 6, 349–385 (1968).