[1] Campanato S.: 
Equazioni ellittiche del II$^o$ ordine e spazi ${\Cal L}^{2,\lambda}$. Ann. Mat. Pura Appl. (4) 69 (1965), 321-381. 
MR 0192168[2] Campanato S.: 
Maggiorazioni interpolatorie negli spazi $H^{m,p}_{\lambda}(Ømega)$. Ann. Mat. Pura Appl. (4) 75 (1967), 261-276. 
MR 0216284[3] Campanato S.: 
Sistemi Ellittici in forma divergenza. Regolarità all' interno. Quaderni Scuola Normale Superiore Pisa, Pisa, 1980. 
MR 0668196 | 
Zbl 0453.35026[4] Campanato S.: 
A maximum principle for non-linear elliptic systems: boundary fundamental estimates. Adv. in Math. 66 (1987), 291-317. 
MR 0915857[5] Cannarsa P.: 
On a maximum principle for elliptic systems with constant coefficients. Rend. Sem. Univ. Padova 64 (1981). 
MR 0636627 | 
Zbl 0473.35016[6] de Giorgi E.: 
Sulla differenziabilitá e l'analiticità delle estremali degli integrali multipli regolari. Memorie dell'Accademia delle Scienze di Torino, Serie 3, 3(I) (1957). 
Zbl 0084.31901[7] de Giorgi E.: 
Un esempio di estremali discontinue per un problema variazionale di tipo ellittico. Boll. Un. Mat. Ital. (4) 1 (1968), 135-137. 
MR 0227827[8] Giaquinta M.: 
Multiple Integrals in the Calculus of Variations and Nonlinear Elliptic Systems. Annals of Math. Studies, Princeton University Press, Princeton, 1983. 
MR 0717034 | 
Zbl 0516.49003[9] Gilbarg D., Trudinger N.S.: 
Elliptic Partial Differential Equations of Second Order. second edition., Springer, 1983. 
MR 0737190 | 
Zbl 1042.35002[10] Kalita E.: 
On the Hölder continuity of solutions of nonlinear parabolic systems. Comment. Math. Univ. Carolinae 35.4 (1994), 675-680. 
MR 1321237 | 
Zbl 0814.35011[11] Koshelev A.I.: 
Regularity Problem for Quasilinear Elliptic and Parabolic Systems. Springer, 1995. 
MR 1442954 | 
Zbl 0847.35023[12] Kottas J.: 
Interior regularity for the quasilinear elliptic systems with nonsmooth coefficients. Comment. Math. Univ. Carolinae 28.1 (1987), 95-102. 
MR 0889771[13] Kottas J.: Regularita slabých řešení eliptických úloh. Thesis of Dissertation, Charles University, Prague, 1990.
[14] Kufner A., John O., Fučík S.: 
Function Spaces. Noordhoff Int. Pub. Leyden, Academia Prague, 1977. 
MR 0482102[15] Leonardi S.: 
On constants of some regularity theorems. De Giorgi's type counterexample. Math. Nachr. 192 (1998). 
MR 1626336 | 
Zbl 0909.35030[16] Leonardi S.: 
Remarks on the regularity of solutions of elliptic systems. Kluwer Academic/ Plenum Publishers, New York, 1999. 
MR 1727457 | 
Zbl 0952.35034[17] Maz'ya V.G.: 
Examples of nonregular solutions of quasilinear elliptic equations with analytic coefficients. Funktsional'nyi Analiz. i Ego Prilosheniya 2 (1968). 
MR 0237946 | 
Zbl 0179.43601[18] Meyers N.G.: 
An $L^p$ estimate for the gradient of solutions of second order elliptic divergence equations. Ann. Scuola Norm. Sup. Pisa, (3) 17 (1963). 
MR 0159110[19] Miranda C.: 
Istituzioni di Analisi Funzionale Lineare. U.M.I. 1978. 
Zbl 0697.46002[20] Nečas J.: 
Les Méthodes Directes en Théorie des Équationes elliptiques. Masson et Cie, Éditeurs, Paris, 1967. 
MR 0227584[21] Nečas J.: On the regularity of weak solutions to nonlinear elliptic systems of partial differential equations. Lectures at Scuola Normale Superiore, Pisa, 1979.
[22] Nečas J.: 
Introduction to the Theory of Nonlinear Elliptic Equations. Teubner-Texte Math., Leipzig, 1983 or J. Wiley, Chichester, 1986. 
MR 0874752[23] Nečas J., Stará J.: 
Principio di massimo per sistemi ellittici quasi lineari non diagonali. Boll. Un. Mat. Ital. (4) 6 (1972), 1-10. 
MR 0315281