Three observations on commutators of Singular Integral Operators with BMO functions

Abstract: This paper contains three observations on commutators of Singular Integral Operators with BMO functions: 1) The subgaussian local decay for the commutator, namely [\frac{1}{|Q|}\left|\left{x\in Q\, : \, |b,T(x)|>M^2f(x)t\right}\right|\leq c e^{-\sqrt{ct|b|_{BMO}}} ] is sharp, that is, it is subgaussian and not better. 2) It is not possible to obtain a pointwise control of the commutator by a finite sum of sparse operators defined with $L\log L$ averages. 3) If $w\in A_p\setminus A_1$ then $\left| wM\left(\frac{f}{w}\right)\right|_{L^1(\mathbb{R}^n)\rightarrow L^{1,\infty}(\mathbb{R}^n)}=\infty$.