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arXiv 0902.1538math.PRmath.CO
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Littlewood-Offord问题的双线性和二次变分

Bilinear and Quadratic Variants on the Littlewood-Offord Problem

Kevin P. Costello

摘要:

如果f(x_1,x_2,...,x_n)是依赖于大量独立伯努利随机变量的多项式,关于f在任何单个值上的最大集中度可以说什么?对于线性多项式,这简化为经典的Littlewood-Offord问题的一个版本:给定非零常数a_1到a_n,形式为+/- a_1 +/- a_2 +/-.的和的最大数目是多少。+/- a_n取任何单个值?这里我们考虑f是双线性型或二次型的情况。对于双线性情形,我们证明了浓度显著大于n^{-1}的唯一形式是那些在某种意义上非常接近简并的形式。对于二次情形,我们证明了没有一个具有许多非零系数的形式具有显著大于n^{-1/2}的浓度。在这两种情况下,结果几乎是紧密的。

英文摘要:

If f(x_1, x_2, ..., x_n) is a polynomial dependent on a large number of independent Bernoulli random variables, what can be said about the maximum concentration of f on any single value? For linear polynomials, this reduces to one version of the classical Littlewood-Offord problem: Given nonzero constants a_1 through a_n, what is the maximum number of sums of the form +/- a_1 +/- a_2 +/-... +/- a_n which take on any single value? Here we consider the case where f is either a bilinear form or a quadratic form. For the bilinear case, we show that the only forms having concentration significantly larger than n^{-1} are those which are in a certain sense very close to being degenerate. For the quadratic case, we show that no form having many nonzero coefficients has concentration significantly larger than n^{-1/2}. In both cases the results are nearly tight.

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