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arXiv 1004.4240cs.DS
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一种稀疏的Johnson-Lindenstrauss变换

A Sparse Johnson--Lindenstrauss Transform

Anirban Dasgupta, Ravi Kumar, Tamás Sarlós

摘要:

降维是一种重要的算法工具,在流模型中有着广泛的应用,包括最近邻搜索、压缩感知和线性代数。在这项工作中,我们得到了{ \em稀疏}版本的基本工具,在维减少-约翰逊-林登施特劳斯变换。使用散列和局部致密化,我们构造了一个稀疏投影矩阵,每列只有$\tilde{O}(\frac{1}ε)$非零条目。我们还显示了一个匹配的下界的稀疏投影矩阵的一大类。我们的边界是有点令人惊讶的,已知的下界$Ω(\frac{1}{ε^2})$的行数的任何投影矩阵和稀疏性的投影矩阵产生的自然建设。使用这一点,我们实现了$(1\pmε)$近似投影的每个非零元素的$\tilde{O}(\frac{1}ε)$更新时间,从而大大优于现有方法所需的$\tilde{O}(\frac{1}{ε^2})$更新时间。我们的方法的一个变体为稀疏向量提供了相同的保证,但其$\tilde{O}(d)$最坏情况下的运行时间与Ailon和Liberty的最佳方法相匹配。

英文摘要:

Dimension reduction is a key algorithmic tool with many applications including nearest-neighbor search, compressed sensing and linear algebra in the streaming model. In this work we obtain a {\em sparse} version of the fundamental tool in dimension reduction --- the Johnson--Lindenstrauss transform. Using hashing and local densification, we construct a sparse projection matrix with just $\tilde{O}(\frac{1}ε)$ non-zero entries per column. We also show a matching lower bound on the sparsity for a large class of projection matrices. Our bounds are somewhat surprising, given the known lower bounds of $Ω(\frac{1}{ε^2})$ both on the number of rows of any projection matrix and on the sparsity of projection matrices generated by natural constructions. Using this, we achieve an $\tilde{O}(\frac{1}ε)$ update time per non-zero element for a $(1\pmε)$-approximate projection, thereby substantially outperforming the $\tilde{O}(\frac{1}{ε^2})$ update time required by prior approaches. A variant of our method offers the same guarantees for sparse vectors, yet its $\tilde{O}(d)$ worst case running time matches the best approach of Ailon and Liberty.

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