Estimates for the covariant derivative of the heat semigroup on differential forms, and covariant Riesz transforms
With $\vec{\Delta}_j\geq 0$ is the uniquely determined self-adjoint realization of the Laplace operator acting on $j$-forms on a geodesically complete Riemannian manifold $M$ and $\nabla$ the Levi-Civita covariant derivative, we prove amongst other things a Li-Yau type heat kernel bound for $\nabla \mathrm{e}^{ -t\vec{\Delta}_j }$, if the curvature tensor of $M$ and its covariant derivative are bounded, an exponentially weighted $L^p$ bound for the heat kernel of $\nabla \mathrm{e}^{ -t\vec{\Delta}_j }$, if the curvature tensor of $M$ and its covariant derivative are bounded, that $\nabla \mathrm{e}^{ -t\vec{\Delta}_j }$ is bounded in $L^p$ for all $1\leq p<\infty$, if the curvature tensor of $M$ and its covariant derivative are bounded, and a second order Davies-Gaffney estimate (in terms of $\nabla$ and $\vec{\Delta}_j$) for $\mathrm{e}^{ -t\vec{\Delta}_j }$ for small times, if the $j$-th degree Bochner-Lichnerowicz potential $V_j=\vec{\Delta}_j-\nabla^{\dagger}\nabla$ of $M$ is bounded from below (where $V_1=\mathrm{Ric}$), which is shown to fail for large times if $V_j$ is bounded. Based on these results, we formulate a conjecture on the boundedness of the covariant local Riesz-transform $\nabla (\vec{\Delta}_j+\kappa)^{-1/2}$ in $L^p$ for all $1\leq p<\infty$ (which we prove for $1\leq p\leq 2$), and explain its implications to geometric analysis, such as the $L^p$-Calder\'on-Zygmund inequality. Our main technical tool is a Bismut derivative formula for $\nabla \mathrm{e}^{ -t\vec{\Delta}_j }$.