[1] M. Giaquinta: Multiple integrals in the calculus of variations and nonlinear elliptic systems. Universität Bonn, Sonderforschungsbereich 72, Preprint, No. 443. MR 0717034 | Zbl 0516.49003

[2] M. Giaquinta J. Nečas: On the regularity of weak solutions to nonlinear elliptic systems of partial differential equations. J. Reine Angew. Math. 316 (1980), 140-159. MR 0581329

[3] D. Gilbarg N. S. Trudinger: Elliptic partial differential equations of second order. Springer-Verlag, New lork, 1977. MR 0473443

[4] P. A. Ivert: Regularitätsuntersuchungen von Lösungen elliptiseher Systeme von quasilinearen Differentialgleichungen zweiter Ordnung. Manuscripta Math.30 (1970), 53-88. MR 0552363

[5] J. Nečas: Les méthodes directes en théorie des équations elliptiques. Academia, Prague, 1967. MR 0227584

[6] J. Nečas: A neceesary and sufficient condition for the regularity of weak solutions to nonlinear elliptic systems of partial differential equations. Abh. Akad. Wiss. DDR, 1981. MR 0639924

[7] J. Nečas: Régularité des solutions faibles d'équations elliptiques nonlinéaires; applications à l'élasticité. Université Paris VI, Analyse numérique et fonctionnelle, 1981.

[8] K. Uhlenbeck: Regularity for a class- of non-linear elliptic systems. Acta Math. 138 (1977), 219-240. MR 0474389