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arXiv 1105.2252math.CAmath.PR
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基于Bellman函数的Haar位移的精确$A_2$估计

Sharp $A_2$ estimates of Haar shifts via Bellman function

Sergei Treil

摘要:

本文利用Bellman函数方法给出了Haar位移的一个尖锐的加权估计的初等证明,该估计在权的$A_2$范数和位移的复杂度中是线性的.连同一般的Calderon-Zygmund算子作为Haar位移的加权平均(在所有并矢格上)的表示,(cf. arXiv:1010.0755v2[math.CA],arXiv:1007.4330v1[math.CA])它给出了所谓的$A_2$猜想的一个明显更简单的证明。主要的估计是关于凹函数的一个非常一般的事实,这对于鞅调和分析的其它问题也是非常有用的。这种类型的凹函数表现为鞅乘子的双线性型上界的Bellman函数,因此主要估计允许将最简单可能鞅乘子的结果转移到更一般的鞅变换上.注意(虽然这对于一般Calderon-Zygmund算子的$A_2$猜想并不重要),这个初等证明给出了移位复杂性的最佳(线性)增长。

英文摘要:

We use the Bellman function method to give an elementary proof of a sharp weighted estimate for the Haar shifts, which is linear in the $A_2$ norm of the weight and in the complexity of the shift. Together with the representation of a general Calderón--Zygmund operator as a weighted average (over all dyadic lattices) of Haar shifts, (cf. arXiv:1010.0755v2[math.CA], arXiv:1007.4330v1[math.CA]) it gives a significantly simpler proof of the so-called the $A_2$ conjecture. The main estimate is a very general fact about concave functions, which can be very useful in other problems of martingale Harmonic Analysis. Concave functions of such type appear as the Bellman functions for bounds on the bilinear form of martingale multipliers, thus the main estimate allows for the transference of the results for simplest possible martingale multipliers to more general martingale transforms. Note that (although this is not important for the $A_2$ conjecture for general Calderón--Zygmund operators) this elementary proof gives the best known (linear) growth in the complexity of the shift.

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