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Viscosity Solutions of Fully Second-Order HJB Equations on Wasserstein Space

A well-posedness result showing that value functions of mean field control problems with common noise are the unique viscosity solutions, in a Crandall-Lions-style framework, of Hamilton-Jacobi-Bellman equations on the Wasserstein space whose second-order derivative in the measure variable is state-dependent and genuinely infinite-dimensional. Existence uses smooth approximations built from finite particle systems; uniqueness rests on a comparison theorem obtained via compactness from penalization of measure moments, and the setting allows unbounded dynamics and state-dependent common-noise volatility.