# Hardy-Littlewood-Sobolev inequalities for a class of non-symmetric and non-doubling hypoelliptic semigroups

## TL;DR

This paper extends Hardy-Littlewood-Sobolev inequalities to a class of non-symmetric, non-doubling hypoelliptic semigroups using semigroup theory and nonlocal calculus, addressing degenerate operators with nonnegative trace.

## Contribution

It introduces new Hardy-Littlewood-Sobolev inequalities for hypoelliptic operators with non-symmetric, non-doubling properties, expanding previous symmetric semigroup results.

## Key findings

- Established Hardy-Littlewood-Sobolev inequalities for non-symmetric hypoelliptic semigroups.
- Demonstrated the applicability of semigroup theory combined with nonlocal calculus in degenerate settings.
- Extended classical inequalities to a broader class of operators with nonnegative trace drift matrices.

## Abstract

In his seminal 1934 paper on Brownian motion and the theory of gases Kolmogorov introduced a second order evolution equation which displays some challenging features. In the opening of his 1967 hypoellipticity paper H\"ormander discussed a general class of degenerate Ornstein-Uhlenbeck operators that includes Kolmogorov's as a special case. In this note we combine semigroup theory with a nonlocal calculus for these hypoelliptic operators to establish new inequalities of Hardy-Littlewood-Sobolev type in the situation when the drift matrix has nonnegative trace. Our work has been influenced by ideas of E. Stein and Varopoulos in the framework of symmetric semigroups. One of our objectives is to show that such ideas can be pushed to successfully handle the present degenerate non-symmetric setting.