Mathematics > Functional Analysis
[Submitted on 1 May 2024]
Title:Variational inequalities for the Ornstein--Uhlenbeck semigroup: the higher--dimensional case
View PDFAbstract:We study the $\varrho$-th order variation seminorm of a general Ornstein--Uhlenbeck semigroup $\left(\mathcal H_t\right)_{t>0}$ in $\mathbb R^n$, taken with respect to $t$. We prove that this seminorm defines an operator of weak type $(1,1)$ with respect to the invariant measure when $\varrho> 2$. For large $t$, one has an enhanced version of the standard weak-type $(1,1)$ bound. For small $t$, the proof hinges on vector-valued Calderón--Zygmund techniques in the local region, and on the fact that the $t$ derivative of the integral kernel of $\mathcal H_t$ in the global region has a bounded number of zeros in $(0,1]$. A counterexample is given for $\varrho= 2$; in fact, we prove that the second order variation seminorm of $\left(\mathcal H_t\right)_{t>0}$, and therefore also the $\varrho$-th order variation seminorm for any $\varrho\in [1,2)$, is not of strong nor weak type $(p,p)$ for any $p \in [1,\infty)$ with respect to the invariant measure.
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