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3. A remark on a comparison theorem of Swanson;Kreith,1969
4. A strong comparison theorem for selfadjoint elliptic equations;Kreith,1968
5. Un teorema sulle soluzioni delle equazioni lineari ellitiche autoaggiunte alle derivate parziali del secondo ordine;Picone;Atti Accad. Naz. Lincei Cl. Sci. Fiz., Mat. Natur. Rend.,1911