1. E.De Giorgi, Sulla differenziabilità e l’analiticità delie estremali degli integrait multipli regolari. Mem. Accad. Sci. Torino cl. Sci. Fis. Mat. Nat. 3 (1957), 25–43.
2. J.Frehse, On two-dimensional quasilinear elliptic systems. Manuscripta math. 28 (1959), 21–50.
3. J.Frehse, Existence and perturbation theorems of nonlinear elliptic systems. From: Nonlinear partial diff. eq. and their applications. Collège de France Seminar IV. Ed. H.Brezis, J.L. Lions, Pitman 1983.
4. M.Giaquinta, Multiple integrals in the calculus of variations and nonlinear elliptic systems. Vorlesungsreihe SFB 72, No.6; Univ. Bonn.
5. M.Giaquinta, E.Giusti, Nonlinear elliptic systems with quadratic growth. Manuscripta math. 24 (1978), 323–349.