2025 Existence of Viscosity Solutions to Abstract Cauchy Problems via Nonlinear Semigroups Fabian Fuchs, and Max Nendel 2025 arXiv 2024 A comparison principle based on couplings of partial integro-differential operators Serena Della Corte, Fabian Fuchs, Richard C. Kraaij, and Max Nendel 2024 arXiv