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. Lecture Notes in Physics, vol. 195;J. Ball,1984