Campanato Spaces via Quantum Markov Semigroups on Finite von Neumann Algebras

Abstract We study the Campanato spaces associated with quantum Markov semigroups on a finite von Neumann algebra $\mathcal M$. Let $\mathcal T=(T_{t})_{t\geq 0}$ be a Markov semigroup, $\mathcal P=(P_{t})_{t\geq 0}$ the subordinated Poisson semigroup and $\alpha>0$. The column Campanato space ${\mathcal{L}^{c}_{\alpha }(\mathcal{P})}$ associated to $\mathcal P$ is defined to be the subset of $\mathcal M$ with finite norm which is given by $$ \begin{align*} \|f\|_{\mathcal{L}^{c}_{\alpha}(\mathcal{P})}=\left\|f\right\|_{\infty}+\sup_{t>0}\frac{1}{t^{\alpha}}\left\|P_{t}|(I-P_{t})^{[\alpha]+1}f|^{2}\right\|^{\frac{1}{2}}_{\infty}. \end{align*} $$The row space ${\mathcal{L}^{r}_{\alpha }(\mathcal{P})}$ is defined in a canonical way. In this article, we will first show the surprising coincidence of these two spaces ${\mathcal{L}^{c}_{\alpha }(\mathcal{P})}$ and ${\mathcal{L}^{r}_{\alpha }(\mathcal{P})}$ for $0<\alpha <2$. This equivalence of column and row norms is generally unexpected in the noncommutative setting. The approach is to identify both of them as the Lipschitz space ${\Lambda _{\alpha }(\mathcal{P})}$. This coincidence passes to the little Campanato spaces $\ell ^{c}_{\alpha }(\mathcal{P})$ and $\ell ^{r}_{\alpha }(\mathcal{P})$ for $0<\alpha <\frac{1}{2}$ under the condition $\Gamma ^{2}\geq 0$. We also show that any element in ${\mathcal{L}^{c}_{\alpha }(\mathcal{P})}$ enjoys the higher-order cancellation property, that is, the index $[\alpha ]+1$ in the definition of the Campanato norm can be replaced by any integer greater than $\alpha $. It is a surprise that this property holds without further condition on the semigroup. Lastly, following Mei’s work on BMO, we also introduce the spaces ${\mathcal{L}^{c}_{\alpha }(\mathcal{T})}$ and explore their connection with ${\mathcal{L}^{c}_{\alpha }(\mathcal{P})}$. All the above-mentioned results seem new even in the (semi-)commutative case.