Gradient Estimates for Neumann Semigroups on Manifolds with Boundary under Unbounded Curvature Conditions

This paper establishes Bismut-type formulas and gradient estimates for Feynman--Kac semigroups on Riemannian manifolds with boundary, under geometric conditions formulated in terms of Ricci curvature $\mathrm{Ric}_Z \geq K$ and second fundamental form $\mathrm{II} \geq \sigma$ for potentially unbounded functions $K$ and $\sigma$.

We then apply these formulas to derive pointwise gradient estimates for the Neumann semigroup under variable, possibly unbounded, lower curvature bounds.

Both convex and non-convex boundary cases are treated.

In the non-convex case, the boundary contribution is controlled by a conformal change of metric and an exponential estimate for the boundary local time.