The $\square_b$ Heat Equation and Multipliers via the Wave Equation

TL;DR

This paper provides a simplified proof of estimates for the $ox_b$-heat equation's kernel on weakly-pseudoconvex domains, extending to multiplier operators using wave equation techniques and Mihlin-H"ormander conditions.

Contribution

It introduces a straightforward proof approach connecting heat and wave equations, generalizes estimates to similar contexts, and analyzes multipliers of $ox_b$ as NIS operators.

Findings

Schwartz kernel of $e^{-tox_b}$ satisfies off-diagonal estimates.

Kernel of $e^{-tox_b}- ext{Szeg"o}$ satisfies on-diagonal estimates.

Multipliers $m(ox_b)$ are shown to be NIS operators under Mihlin-H"ormander conditions.

Abstract

Recently, Nagel and Stein studied the $\square_b$-heat equation, where $\square_b$ is the Kohn Laplacian on the boundary of a weakly-pseudoconvex domain of finite type in $\C^2$. They showed that the Schwartz kernel of $e^{-t\square_b}$ satisfies good "off-diagonal" estimates, while that of $e^{-t\square_b}-\pi$ satisfies good "on-diagonal" estimates, where $\pi$ is the Szeg\"o projection. We offer a simple proof of these results, which easily generalizes to other, similar situations. Our methods involve adapting the well-known relationship between the heat equation and the finite propagation speed of the wave equation to this situation. In addition, we apply these methods to study multipliers of the form $m\l(\square_b\r)$. In particular, we show that $m\l(\square_b\r)$ is an NIS operator, where $m$ satisfies an appropriate Mihlin-H\"ormander condition.