Maximum principle for viscosity sub solutions and viscosity sub solutions of the Laplacian. (English) Zbl 0822.35019
Atti Accad. Naz. Lincei, Cl. Sci. Fis. Mat. Nat., IX. Ser., Rend. Lincei, Mat. Appl. 4, No. 3, 213-217 (1993).