On time-dependent Besov vector fields and the regularity of their flows

Abstract: We show ODE-closedness for a large class of Besov spaces $\mathcal{B}^{n,\alpha,p}(\mathbb{R}^d,\mathbb{R}^d)$, where $n \geq 1,~\alpha \in (0,1],~p \in [1,\infty]$. ODE-closedness means that pointwise time-dependent $\mathcal{B}^{n,\alpha,p}$-vector fields $u$ have unique flows $\Phi_u \in \operatorname{Id} + \mathcal{B}^{n,\alpha,p}(\mathbb{R}^d,\mathbb{R}^d)$. The class of vector fields under consideration contains as a special case the class of Bochner integrable vector fields $L^1(I, \mathcal{B}^{n,\alpha,p}(\mathbb{R}^d,\mathbb{R}^d))$. In addition, for $n \geq 2$ and $\alpha < \beta$, we show continuity of the flow mapping $L^1(I,\mathcal{B}^{n,\beta,p}(\mathbb{R}^d,\mathbb{R}^d)) \rightarrow C(I,\mathcal{B}^{n,\alpha,p}(\mathbb{R}^d,\mathbb{R}^d)), ~ u \mapsto \Phi_u-\operatorname{Id}$. We even get $\gamma$-H\"older continuity for any $\gamma < \beta - \alpha$.