Iterated commutators of multilinear Calderon-Zygmund operators and pointwise
multiplication with functions in $BMO$ are studied in products of Lebesgue
spaces. Both strong type and weak end-point estimates are obtained, including
weighted results involving the vectors weights of the multilinear
Calderon-Zygmund theory recently introduced in the literature. Some better than
expected estimates for certain multilinear operators are presented too.
We show that if an operator T is bounded on weighted Lebesgue space L^2(w)
and obeys a linear bound with respect to the A_2 constant of the weight, then
its commutator [b,T] with a function b in BMO will obey a quadratic bound with
respect to the A_2 constant of the weight. We also prove that the kth-order
commutator T^k_b=[b,T^{k-1}_b] will obey a bound that is a power (k+1) of the
A_2 constant of the weight. Sharp extrapolation provides corresponding L^p(w)
estimates.
A multivariable version of the strong maximal function is introduced and a
sharp distributional estimate for this operator in the spirit of the Jessen,
Marcinkiewicz, and Zygmund theorem is obtained. Conditions that characterize
the boundedness of this multivariable operator on products of weighted Lebesgue
spaces equipped with multiple weights are obtained. Results for other
multi(sub)linear maximal functions associated with bases of open sets are
studied too.
We give a new proof of the sharp one weight $L^p$ inequality for any operator
$T$ that can be approximated by Haar shift operators such as the Hilbert
transform, any Riesz transform, the Beurling-Ahlfors operator. Our proof avoids
the Bellman function technique and two weight norm inequalities. We use instead
a recent result due to A. Lerner to estimate the oscillation of dyadic
operators.