no. 1
A comparison theorem for nonlinear operators
Annali della Scuola Normale Superiore di Pisa - Scienze Fisiche e Matematiche, Série 3, Tome 25 (1971) no. 1, pp. 41-46. 
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     author = {Allegretto, W.},
     title = {A comparison theorem for nonlinear operators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Scienze Fisiche e Matematiche},
     pages = {41--46},
     publisher = {Scuola normale superiore},
     volume = {Ser. 3, 25},
     number = {1},
     year = {1971},
     mrnumber = {298181},
     zbl = {0225.35013},
     language = {en},
     url = {http://archive.numdam.org/item/ASNSP_1971_3_25_1_41_0/}
}
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Allegretto, W. A comparison theorem for nonlinear operators. Annali della Scuola Normale Superiore di Pisa - Scienze Fisiche e Matematiche, Série 3, Tome 25 (1971) no. 1, pp. 41-46. http://archive.numdam.org/item/ASNSP_1971_3_25_1_41_0/

[1] W. Allegretto, PhD dissertation, University of British Columbia, 1969.

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K. Kreith; A comparison theorem for general elliptic equations with mixed boundary conditions, J. Diff. Eq. 8 (3), 1970, pp. 537-541. | MR | Zbl

D. Dunninger; A. Picone identity for non-self-adjoint elliptic operators, Atti, Accad. Naz. Lincei, Cl. Sci, Fiz, Mat. Natur. Rend. XLVIII (1970), 133-139. | MR | Zbl

G. Platone and G. Maria; Teoremi di confronto per soprasoluzioni rispetto a operatori ellittici del secondo ordine a coefficienti discontinui in aperti illimitati, Boll. U. M. I., IV Ser. 3, (1970), pp, 362-374. | MR | Zbl