Lipschitz estimates in quasi-Banach Schatten ideals

Abstract: We study the class of functions $f$ on $\mathbb{R}$ satisfying a Lipschitz estimate in the Schatten ideal $\mathcal{L}p$ for $0 < p \leq 1$. The corresponding problem with $p\geq 1$ has been extensively studied, but the quasi-Banach range $0 < p < 1$ is by comparison poorly understood. Using techniques from wavelet analysis, we prove that Lipschitz functions belonging to the homogeneous Besov class $\dot{B}^{\frac{1}{p}}{\frac{p}{1-p},p}(\mathbb{R})$ obey the estimate $$ |f(A)-f(B)|{p} \leq C{p}(|f'|{L{\infty}(\mathbb{R})}+|f|{\dot{B}^{\frac{1}{p}}{\frac{p}{1-p},p}(\mathbb{R})})|A-B|{p} $$ for all bounded self-adjoint operators $A$ and $B$ with $A-B\in \mathcal{L}_p$. In the case $p=1$, our methods recover and provide a new perspective on a result of Peller that $f \in \dot{B}^1{\infty,1}$ is sufficient for a function to be Lipschitz in $\mathcal{L}1$. We also provide related H\"older-type estimates, extending results of Aleksandrov and Peller. In addition, we prove the surprising fact that non-constant periodic functions on $\mathbb{R}$ are not Lipschitz in $\mathcal{L}_p$ for any $0 < p < 1$. This gives counterexamples to a 1991 conjecture of Peller that $f \in \dot{B}^{1/p}{\infty,p}(\mathbb{R})$ is sufficient for $f$ to be Lipschitz in $\mathcal{L}_p$.