“…Such equations may not possess smooth solutions (not even in the linear case), and the appropriate notion of solutions is that of viscosity solutions. The theory of viscosity solutions for (pure) partial differential equations is highly developed [24,31], and there has been an interest in recent years to apply this theory to integro-partial differential equations [1,2,3,4,5,7,14,15,16,32,37,38,47,48,49,51,52,54,55]. Of particular relevance to the present paper are the works [37,38].…”