Giaquinta is well known for his basic work in elliptic regularity theory, and especially in the setting of vectorial variational problems. Together with Enrico Giusti he has obtained innovative results[4][5][6] on the regularity of minima of variational integrals and related singular sets. The main novelty is in the fact that, for the first time, the regularity of minimizers is obtained using directly the minimality properties without appealing to Euler–Lagrange equation of the functionals, which in general is not supposed to exist in the cases considered. His work with Giuseppe Modica on the local higher integrability properties of solutions to elliptic systems has had an influence on the development of partial regularity theory.[7] Many of these results are summarized in his 1983 book.[8]
Giaquinta has been one of the founders, and for many years the managing editor, of the journal "Calculus of Variations and PDE".
Giaquinta, Mariano; Hildebrandt, Stefan (1996), Calculus of Variations I. The Lagrangian Formalism, Grundlehren der Mathematischen Wissenschaften, vol. 310 (1st ed.), Berlin: Springer–Verlag, pp. xxix+475, ISBN3-540-50625-X, MR1368401, Zbl0853.49001.
Giaquinta, Mariano; Hildebrandt, Stefan (1996), Calculus of Variations II. The Hamiltonian Formalism, Grundlehren der Mathematischen Wissenschaften, vol. 311 (1st ed.), Berlin: Springer–Verlag, pp. xxx+652, ISBN3-540-57961-3, MR1385926, Zbl0853.49002.
^M. Giaquinta, E. Giusti: "On the regularity of the minima of variational integrals" in Acta Mathematica 148 (1982), 31–46
^M. Giaquinta, E. Giusti: "Differentiability of minima of nondifferentiable functionals" in Inventiones Mathematicae 72 (1983), 285–298
^M. Giaquinta, E. Giusti: "The singular set of the minima of certain quadratic functionals" in Annali della Scuola Normale Superiore di Pisa Classe di Scienze (Serie 4) 11 (1984), 45–55
^M. Giaquinta, G. Modica: "Regularity results for some classes of higher order nonlinear elliptic systems" in Journal fuer die Reine und Angewandte Mathematik (Crelles J.), 311/312 (1979), pp145-169
^M. Giaquinta, "Multiple integrals in the calculus of variations and nonlinear elliptic systems", in Annals of Mathematics Studies, 105. Princeton University Press, Princeton, New Jersey, 1983. ISBN0-691-08330-4; 0-691-08331-2