Linear Forms and Higher-Degree Uniformity for Functions On $${mathbb{F}^{n}_{p}}$$
Linear Forms and Higher-Degree Uniformity for Functions On $${mathbb{F}^{n}_{p}}$$
复制标题
$${mathbb{F}^{n}_{p}}$$ 上函数的线性形式和更高阶均匀性
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
J. Wolf
中科院分区：
文献类型：
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作者：
W. Gowers;J. Wolf
In [GW1] we began an investigation of the following general question. Let L1, . . . , Lm be a system of linear forms in d variables on $${F^n_p}$$, and let A be a subset of $${F^n_p}$$ of positive density. Under what circumstances can one prove that A contains roughly the same number of m-tuples L1(x1, . . . , xd), . . . , Lm(x1, . . . , xd) with $${x_1,ldots, x_d in {mathbb F}^n_p}$$ as a typical random set of the same density? Experience with arithmetic progressions suggests that an appropriate assumption is that $${||A - delta 1||_{U{^k}}}$$ should be small, where we have written A for the characteristic function of the set A, δ is the density of A, k is some parameter that depends on the linear forms L1, . . . , Lm, and $${|| cdot ||_U{^k}}$$ is the kth uniformity norm. The question we investigated was how k depends on L1, . . . , Lm. Our main result was that there were systems of forms where k could be taken to be 2 even though there was no simple proof of this fact using the Cauchy–Schwarz inequality. Based on this result and its proof, we conjectured that uniformity of degree k − 1 is a sufficient condition if and only if the kth powers of the linear forms are linearly independent. In this paper we prove this conjecture, provided only that p is sufficiently large. (It is easy to see that some such restriction is needed.) This result represents one of the first applications of the recent inverse theorem for the Uk norm over $${F^n_p}$$ by Bergelson, Tao and Ziegler [TZ2], [BTZ]. We combine this result with some abstract arguments in order to prove that a bounded function can be expressed as a sum of polynomial phases and a part that is small in the appropriate uniformity norm. The precise form of this decomposition theorem is critical to our proof, and the theorem itself may be of independent interest.
In [GW1] we began an investigation of the following general question. Let L1, . . . , Lm be a system of linear forms in d variables on $${F^n_p}$$, and let A be a subset of $${F^n_p}$$ of positive density. Under what circumstances can one prove that A contains roughly the same number of m-tuples L1(x1, . . . , xd), . . . , Lm(x1, . . . , xd) with $${x_1,ldots, x_d in {mathbb F}^n_p}$$ as a typical random set of the same density? Experience with arithmetic progressions suggests that an appropriate assumption is that $${||A - delta 1||_{U{^k}}}$$ should be small, where we have written A for the characteristic function of the set A, δ is the density of A, k is some parameter that depends on the linear forms L1, . . . , Lm, and $${|| cdot ||_U{^k}}$$ is the kth uniformity norm. The question we investigated was how k depends on L1, . . . , Lm. Our main result was that there were systems of forms where k could be taken to be 2 even though there was no simple proof of this fact using the Cauchy–Schwarz inequality. Based on this result and its proof, we conjectured that uniformity of degree k − 1 is a sufficient condition if and only if the kth powers of the linear forms are linearly independent. In this paper we prove this conjecture, provided only that p is sufficiently large. (It is easy to see that some such restriction is needed.) This result represents one of the first applications of the recent inverse theorem for the Uk norm over $${F^n_p}$$ by Bergelson, Tao and Ziegler [TZ2], [BTZ]. We combine this result with some abstract arguments in order to prove that a bounded function can be expressed as a sum of polynomial phases and a part that is small in the appropriate uniformity norm. The precise form of this decomposition theorem is critical to our proof, and the theorem itself may be of independent interest.