The L^p-to-L^q boundedness of commutators with applications to the Jacobian operator

Supplying the missing necessary conditions, we complete the characterisation of the $L^p\to L^q$ boundedness of commutators $[b,T]$ of pointwise multiplication and Calder\'on-Zygmund operators, for arbitrary pairs of $1<p,q<\infty$ and under minimal non-degeneracy hypotheses on $T$. For $p\leq q$ (and especially $p=q$), this extends a long line of results under more restrictive assumptions on $T$. In particular, we answer a recent question of Lerner, Ombrosi, and Rivera-R\'ios by showing that $b\in BMO$ is necessary for the $L^p$-boundedness of $[b,T]$ for any non-zero homogeneous singular integral $T$. We also deal with iterated commutators and weighted spaces. For $p>q$, our results are new even for special classical operators with smooth kernels. As an application, we show that every $f\in L^p(R^d)$ can be represented as a convergent series of normalised Jacobians $Ju=\det\nabla u$ of $u\in \dot W^{1,dp}(R^d)^d$. This extends, from $p=1$ to $p>1$, a result of Coifman, Lions, Meyer and Semmes about $J:\dot W^{1,d}(R^d)^d\to H^1(R^d)$, and supports a conjecture of Iwaniec about the solvability of the equation $Ju=f\in L^p(R^d)$.