1. Yang, K.: The theory of Lie derivatives and its applications. Amsterdam: North-Holland 1957.
2. Hestenes, M.E.: Calculus of variations and optimal control theory. New York: John Wiley 1966.
3. Edelen, D.G.B.: Internat. J. Engrg. Sci. 7, 373–389 (1969).
4. Edelen, D.G.B.: Internat. J. Engrg Sci. 7, 391–399 (1969).
5. Edelen, D.G.B.: Nonlocal variations and local invariance of fields. New York: American Elsevier 1969.