1. Hestenes, M. R.,Calculus of Variations and Optimal Control Theory, pp. 375?387, John Wiley and Sons, New York, 1966.
2. Guinn, T.,On First-Order Necessary Conditions for Variational and Optimal Control Problems, University of California at Los Angeles, Ph.D. Thesis, 1964.
3. Graves, L. M.,Theory of Function of Real Variables, Chapter 9, McGraw-Hill Book Company, New York, 1946.