Published online by Cambridge University Press: 29 November 2019
We show that, if $b\in L^{1}(0,T;L_{\operatorname{loc}}^{1}(\mathbb{R}))$ has a spatial derivative in the John–Nirenberg space
$\operatorname{BMO}(\mathbb{R})$, then it generates a unique flow
$\unicode[STIX]{x1D719}(t,\cdot )$ which has an
$A_{\infty }(\mathbb{R})$ density for each time
$t\in [0,T]$. Our condition on the map
$b$ is not only optimal but also produces a sharp quantitative estimate for the density. As a killer application we achieve the well-posedness for a Cauchy problem of the transport equation in
$\operatorname{BMO}(\mathbb{R})$.