Some obstacles in characterising the boundedness of bi-parameter singular integrals

Abstract

The famous $T1$ theorem for classical Calderón-Zygmund operators is a characterisation for their boundedness in $L^2$. In the bi-parameter case, on the other hand, the current $T1$ theorem is merely a collection of sufficient conditions. This difference in mind, we study a particular dyadic bi-parameter singular integral operator, namely the full mixed bi-parameter paraproduct $P$, which is precisely the operator responsible for the outstanding problems in the bi-parameter theory. We make several remarks about $P$, the common theme of which is to demonstrate the delicacy of the problem of finding a completely satisfactory product $T1$ theorem. For example, $P$ need not be unconditionally bounded if it is conditionally bounded -- a major difference compared to the corresponding one-parameter model operators. Moreover, currently the theory even lacks a characterisation for the potentially easier unconditional boundedness. The product BMO condition is sufficient, but far from necessary: we show by example that unconditional boundedness does not even imply the weaker rectangular BMO condition.

10 pages

Cited by in corpus (7)

• Weighted little bmo and two-weight inequalities for Journé commutators
• Bilinear Calderón-Zygmund theory on product spaces
• Upper Bound for Multi-parameter Iterated Commutators
• Multi-parameter estimates via operator-valued shifts
• Commutators of bilinear bi-parameter singular integrals
• Genuinely multilinear weighted estimates for singular integrals in product spaces
• Higher order Journe commutators and characterizations of multi-parameter BMO