F. Leonetti, E. Mascolo, F. Siepe: Gradient regularity for minimizers of functionals under $p-q$ subquadratic growth " Bollettino UMI Vol. (8), N. 4-B, p. 571-586, 2001 F. Leonetti, E. Mascolo, F. Siepe: Everywhere regularity for a class of vectorial functionals under subquadratic general growth conditions J. Math. Anal. Appl. Vol. 287 , p. 593 - 608 , 2003