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arXiv 2409.07962math.COmath.NT
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关于二次水平集上Gowers $U^3$-范数的一个逆定理

An inverse theorem for the Gowers $U^3$-norm relative to quadratic level sets

Sean Prendiville

AI总结:

该研究证明了有限向量空间高秩二次水平集上Gowers $U^3$-范数的有效逆定理,可用于密度增量论证,得到Brauer四元组Ramsey数指数界等结果,为相关组合数论问题提供新方法。

摘要:

我们证明了有限向量空间中支撑在高秩二次水平集上的函数的Gowers $U^3$-范数逆定理的有效版本。对于受$U^3$-范数控制的构型(复杂度为2的构型),这使得人们可以针对二次水平集开展密度增量论证,而二次水平集是有限向量空间上二次傅里叶分析语境下Bohr集的类似物。我们通过推导三项等差数列与其公差同色的构型(即“Brauer四元组”)的Ramsey数的指数界,展示了这一论证方法,这一结果我们无法通过其他方法得到。\n我们的方法还给出了不含复杂度为2的平移不变构型的集合的密度的多重对数界。Green和Tao曾使用更简单的弱正则性论证得到了四项等差数列的这类界。在附录中,我们给出了一个如何将Green和Tao的论证推广到其他复杂度为2的平移不变构型的示例。不过,这一推广关键依赖于由Croot-Lev-Pach多项式方法得到的一个估计,该估计可能并不适用于所有复杂度为2的系统。因此,针对二次水平集开展密度增量论证对此类问题可能仍然有用。它也可以作为在更一般的幂零Bohr集上开展密度增量的模型,以期实现其他Szemerédi型定理的有效化。

英文摘要:

We prove an effective version of the inverse theorem for the Gowers $U^3$-norm for functions supported on high-rank quadratic level sets in finite vector spaces. For configurations controlled by the $U^3$-norm (complexity-two configurations), this enables one to run a density increment argument with respect to quadratic level sets, which are analogues of Bohr sets in the context of quadratic Fourier analysis on finite vector spaces. We demonstrate such an argument by deriving an exponential bound on the Ramsey number of three-term progressions which are the same colour as their common difference (``Brauer quadruples''), a result we have been unable to establish by other means. Our methods also yield polylogarithmic bounds on the density of sets lacking translation-invariant configurations of complexity two. Such bounds for four-term progressions were obtained by Green and Tao using a simpler weak-regularity argument. In an appendix, we give an example of how to generalise Green and Tao's argument to other translation-invariant configurations of complexity two. However, this crucially relies on an estimate coming from the Croot-Lev-Pach polynomial method, which may not be applicable to all systems of complexity two. Hence running a density increment with respect to quadratic level sets may still prove useful for such problems. It may also serve as a model for running density increments on more general nil-Bohr sets, with a view to effectivising other Szemerédi-type theorems.

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