Lifting Partial Smoothing to Solve HJB Equations and Stochastic Control Problems

We study a family of stochastic control problems arising in typical applications (such as boundary control and control of delay equations with delay in the control) with the ultimate aim of finding solutions of the associated HJB equations, regular enough to find optimal feedback controls. These problems are difficult to treat since the underlying transition semigroups do not possess good smoothing properties or the so-called structure condition, which typically allows to apply the backward equations approach. In Gozzi and Masiero [SIAM J. Control Optim., 55 (2017), pp. 2981-3012; SIAM J. Control Optim., 55 (2017), pp. 3013-3038] and, more recently, Gozzi and Masiero [SIAM J. Control Optim., 61 (2023), pp. 586-619], we studied such problems, developing new partial smoothing techniques which allowed us to obtain the required regularity in the case when the cost functional is independent of the state variable. This is a somehow strong restriction which is not verified in most applications. In this paper (which can be considered a continuation of the research of the above papers), we develop a new approach to overcome this restriction. We extend the partial smoothing result to a wider class of functions which depend on the whole trajectory of the underlying semigroup, and we use this as a key tool to improve our regularity result for the HJB equation. The fact that such class depends on trajectories requires a nontrivial technical work, as we have to lift the original transition semigroup to a space of trajectories, defining a new ``high-level"" environment where our problems can be solved.