On Positive L? Solutions of a Class of Elliptic Systems. W. Allegretto — 1986 Mathematische Zeitschrift
On the Principal Eigenvalues of Indefinite Elliptic Problems. W. Allegretto — 1987 Mathematische Zeitschrift
A comparison theorem for nonlinear operators W. Allegretto — 1971 Annali della Scuola Normale Superiore di Pisa - Classe di Scienze
Boundary problems of the second order with an indefinite weight-function. W. Allegretto; A.B. Mingarelli — 1989 Journal für die reine und angewandte Mathematik