1. T. Angell, A note on approximation of optimal solutions of free problems of the calculus of variations, Rend. Circ. Mat. Palermo 28 (1979), 258–272.
2. J. Ball, Constitutive inequalities and existence theorems in nonlinear elastostatics, in Nonlinear Analysis and Mechanics: Heriot-Watt Symposium, vol. 1, pp. 187–241, Ed. R. J. Knops. Pitman, London, 1977.
3. J. Ball, Discontinuous equilibrium solutions and cavitation in nonlinear elasticity, Phil. Trans. R. Soc. Lond. A 306 (1982), 557–611.
4. J. Ball, Editor, Systems of Nonlinear Partial Differential Equations, Reidel, Dordrecht, 1983.
5. J. Ball, Minimizers and the Euler-Lagrange equations, in Trends and Applications of Pure Mathematics to Mechanics, ed. P. G. Ciarlet & M. Roseau, Lecture Notes in Physics, vol. 195, Springer-Verlag, Berlin Heidelberg New York Tokyo 1984.