The ${\square }_b$ Heat Equation and Multipliers via the Wave Equation

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 Szegö 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 $mł({\square }_b\mathring{)}$. In particular, we show that $mł({\square }_b\mathring{)}$ is an NIS operator, where $m$ satisfies an appropriate Mihlin-Hörmander condition.

29 pages; minor corrections