1. Sewell, M. J.: The governing equations and extremum principles of elasticity and plasticity generated from a single functional I/II. J. Struct. Mech.2, 1?32, 135?158 (1973).
2. Nobel, B., Sewell, M. J.: On dual extremum principles in applied mathematics. J. Inst. Math. Applics.9, 123?193 (1972).
3. Stumpf, H.: Generating functionals and extremum principles in nonlinear elasticity with applications to nonlinear plate and shallow shell theory, in: Lectures Notes in Mathematics 503, S. 500?510. Berlin-Heidelberg-New York: Springer. 1976.
4. Tonti, E.: A mathematical model for physical theories I/II. Rend. Accad. Lincei, Sci. fis. mat. e nat.52, 175?181, 244?250 (1972).
5. Tonti, E.: On the mathematical structure of a large class of physical theories. Rend. Accad. Lincei, Sci. fis. mat. e nat.52, 48?56 (1972).