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arXiv 2101.09592math.COcs.CC
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点-超平面关联几何与对数秩猜想

Point-hyperplane incidence geometry and the log-rank conjecture

Noah Singer, Madhu Sudan

AI总结:

本文从点-超平面关联几何角度提出并证明一个新猜想与对数秩猜想等价,同时改进无结构构型中完全二部子图密度下界与上界构造,为理解改进Lovett上界的困难提供新框架。

摘要:

我们从点-超平面关联几何的视角研究对数秩猜想。我们提出如下猜想:给定 $\mathbb{R}^d$ 中一个被常数大小的平行超平面集合覆盖的点集,存在一个仿射子空间,它占据了关联的很大一部分(即 $2^{-{\operatorname{polylog}(d)}}$ 比例)。或者,我们的猜想可以从线性代数角度解释如下:任何秩为 $d$ 且每列至多包含 $O(1)$ 个不同元素的矩阵,都包含一个分数大小为 $2^{-{\operatorname{polylog}(d)}}$ 的子矩阵,其中每列只包含一个不同元素。我们证明该猜想等价于对数秩猜想。受上述联系的启发,我们重新审视了点-超平面关联几何中在没有结构假设(即分拆的存在性)情况下的若干被广泛研究的问题。我们给出一个初等论证,证明在任何关联密度为 $ε$ 的 $d$ 维构型中,存在密度为 $Ω(ε^{2d}/d)$ 的完全二部子图。我们还改进了 Apfelbaum 和 Sharir(SIAM J. Discrete Math. '07)的上界构造,得到一个完全二部子图指数级小且关联密度为 $Ω(1/\sqrt d)$ 的构型。最后,我们讨论多种构造(来自其他作者),这些构造给出关联密度为 $Ω(1)$ 且二部子图密度为 $2^{-Ω(\sqrt d)}$ 的构型。我们的框架和结果可能有助于阐明改进 Lovett 关于对数秩猜想的 $\tilde{O}(\sqrt{\operatorname{rank}(f)})$ 上界(J. ACM '16)的困难所在;特别地,对该上界的任何改进都将意味着平行 $3$-分拆构型上第一批优于我们对无结构构型通用上界的二部子图大小上界。

英文摘要:

We study the log-rank conjecture from the perspective of point-hyperplane incidence geometry. We formulate the following conjecture: Given a point set in $\mathbb{R}^d$ that is covered by constant-sized sets of parallel hyperplanes, there exists an affine subspace that accounts for a large (i.e., $2^{-{\operatorname{polylog}(d)}}$) fraction of the incidences. Alternatively, our conjecture may be interpreted linear-algebraically as follows: Any rank-$d$ matrix containing at most $O(1)$ distinct entries in each column contains a submatrix of fractional size $2^{-{\operatorname{polylog}(d)}}$, in which each column contains one distinct entry. We prove that our conjecture is equivalent to the log-rank conjecture. Motivated by the connections above, we revisit well-studied questions in point-hyperplane incidence geometry without structural assumptions (i.e., the existence of partitions). We give an elementary argument for the existence of complete bipartite subgraphs of density $Ω(ε^{2d}/d)$ in any $d$-dimensional configuration with incidence density $ε$. We also improve an upper-bound construction of Apfelbaum and Sharir (SIAM J. Discrete Math. '07), yielding a configuration whose complete bipartite subgraphs are exponentially small and whose incidence density is $Ω(1/\sqrt d)$. Finally, we discuss various constructions (due to others) which yield configurations with incidence density $Ω(1)$ and bipartite subgraph density $2^{-Ω(\sqrt d)}$. Our framework and results may help shed light on the difficulty of improving Lovett's $\tilde{O}(\sqrt{\operatorname{rank}(f)})$ bound (J. ACM '16) for the log-rank conjecture; in particular, any improvement on this bound would imply the first bipartite subgraph size bounds for parallel $3$-partitioned configurations which beat our generic bounds for unstructured configurations.

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