Abstract: | We present a theory of well-posedness and a priori estimates for bounded distributional (or very weak) solutions of (0.1) where φ is merely continuous and nondecreasing, and is the generator of a general symmetric Lévy process. This means that can have both local and nonlocal parts like, e.g., . New uniqueness results for bounded distributional solutions to this problem and the corresponding elliptic equation are presented and proven. A key role is played by a new Liouville type result for . Existence and a priori estimates are deduced from a numerical approximation, and energy-type estimates are also obtained. |