Reverse inequalities for super-Riesz transforms on graphs with a slow diffusion

Abstract

In the $D$-dimensional Vicsek graph, we prove that the Riesz-like inequality $\Vert \nabla f{\Vert }_p\le C\Vert \hat{I}{”}^{3}f{\Vert }_p$ holds for every $p\in (1,\infty )$ and every $ 0<γ<γ^*(p):=\frac{1}{D+1}+\frac{D-1}{D+1}\,\frac{1}{p}, $ while it fails whenever $p\in (1,\infty )$ and $γ^*(p)<γ<1$. Thus, the validity of the inequality remains open only at the critical exponent $γ=γ^*(p)$. This provides the first example of an $L^p$-bounded ``super-Riesz transform'', namely an operator of the form $\nabla \hat{I}{”}^{-{\hat{I}}^3}$ with ${\hat{I}}^3$ strictly larger than the Euclidean threshold $\frac{1}{2}$. To achieve this, we establish a more general result linking the diffusion escape rate and a Poincaré inequality on balls to the validity of the reverse Riesz-like inequality $\Vert \hat{I}{”}^{3}f{\Vert }_p\le C\Vert \nabla f{\Vert }_p.$

33 pages