# Domains of pseudo-differential operators: a case for the Triebel--Lizorkin spaces

## TL;DR

This paper establishes optimal continuity results for type 1,1 pseudo-differential operators on Triebel--Lizorkin spaces, extending classical conditions and correcting previous inaccuracies in the literature.

## Contribution

It introduces a new framework for analyzing pseudo-differential operators on Triebel--Lizorkin spaces using paradifferential methods, improving upon prior approaches with reduced symbols.

## Key findings

- Operators of type 1,1 are continuous from F^d_{p,1} to L_p for 1≤p<∞.
- The continuity results are shown to be optimal within the Besov and Triebel--Lizorkin scales.
- The paper corrects and clarifies existing results in the literature regarding these operators.

## Abstract

The main result is that every pseudo-differential operator of type 1,1 and order $d$ is continuous from the Triebel--Lizorkin space $F^d_{p,1}$ to $L_p$, $1\le p<\infty$, and that this is optimal within the Besov and Triebel--Lizorkin scales.The proof also leads to the known continuity for $s>d$, while for all real $s$ the sufficiency of H\"ormander's condition on the twisted diagonal is carried over to the Besov and Triebel--Lizorkin framework. To obtain this, type 1,1-operators are extended to distributions with compact spectrum, and Fourier transformed operators of this type are on such distributions proved to satisfy a support rule, generalising the rule for convolutions. Thereby the use of reduced symbols, as introduced by Coifman and Meyer, is replaced by direct application of the paradifferential methods. A few flaws in the literature have been detected and corrected.