Vertical and horizontal Square Functions on a Class of Non-Doubling Manifolds

TL;DR

This paper investigates the boundedness of vertical and horizontal square functions on a class of non-doubling manifolds formed by connected sums, revealing their different behaviors and implications for Hardy spaces.

Contribution

It establishes boundedness and unboundedness results for vertical and horizontal square functions on non-doubling manifolds, extending harmonic analysis tools to these complex geometric settings.

Findings

Vertical square function bounded on L^p for 1<p<n_min

Horizontal square function bounded on L^p for all 1<p<∞

Vertical and horizontal square functions are not equivalent for p≥n_min

Abstract

We consider a class of non-doubling manifolds $\mathcal{M}$ that are the connected sum of a finite number of $N$-dimensional manifolds of the form $\mathbb{R}^{n_{i}} \times \mathcal{M}_{i}$. Following on from the work of Hassell and the second author \cite{hs2019}, a particular decomposition of the resolvent operators $(\Delta + k^{2})^{-M}$, for $M \in \mathbb{N}^{*}$, will be used to demonstrate that the vertical square function operator $$ Sf(x) := \left( \int^{\infty}_{0} \left|t \nabla (I + t^{2} \Delta)^{-M}f(x)\right|{2} \frac{dt}{t}\right)^{\frac{1}{2}} $$ is bounded on $L^{p}(\mathcal{M})$ for $1 < p < n_{min} = \min_{i} n_{i}$ and weak-type $(1,1)$. In addition, it will be proved that the reverse inequality $\left\Vert f \right\Vert_{p} \lesssim \left\Vert S f\right\Vert_{p}$ holds for $p \in (n_{min}',n_{min})$ and that $S$ is unbounded for $p \geq n_{min}$ provided $2 M < n_{min}$. Similarly, for $M > 1$, this method of proof will also be used to ascertain that the horizontal square function operator $$ sf(x) := \left(\int^{\infty}_{0} \left|t^{2}\Delta (I + t^{2} \Delta)^{-M}f(x)\right|^{2} \, \frac{dt}{t}\right)^{\frac{1}{2}} $$ is bounded on $L^{p}(\mathcal{M})$ for all $1 < p < \infty$ and weak-type $(1,1)$. Hence, for $p \geq n_{min}$, the vertical and horizontal square function operators are not equivalent and their corresponding Hardy spaces $H^p$ do not coincide.