Unexpected combinatorial property of all positive measures on the unit square and unit cube
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Paraproducts are building blocks of many singular integral operators and the main instrument in proving “Leibniz rule” for fractional derivatives (Kato–Ponce). Also multi-parameter paraproducts appear naturally in questions of embedding of spaces of analytic functions in polydisc into Lebesgue spaces with respect to a measure in the polydisc. The latter problem (without loss of information) can be often reduced to boundedness of weighted dyadic multi-parameter paraproducts. We find the necessary and sufficient condition for this boundedness in n-parameter case, when n is 1, 2, or 3. The answer is quite unexpected—it is a certain combinatorial property of all measures in dimension 2 and 3—and seemingly goes against the well known difference between box and Chang–Fefferman condition that was given by Carleson quilts example of 1974.