1. Bliss, G. A.,Lectures on the Calculus of Variations, University of Chicago Press, Chicago, 1946.
2. Hestenes, M. R.,Calculus of Variations and Optimal Control Theory, Chapter 6, John Wiley and Sons, New York, 1966.
3. Hestenes, M. R.,On Variational Theory and Optimal Control Theory, SIAM Journal on Control, Vol. 3, pp. 23–48, 1965.
4. Oguztóreli, M. N.,Time-Lag Control Systems, Academic Press, New York, 1966.
5. Chyung, D. H., andLee, E. B.,Linear Optimal Systems with Time Delay, SIAM Journal on Control, Vol. 4, pp. 548,575, 1966.