Near Optimal Dimensionality Reductions That Preserve Volumes

  • Authors:
  • Avner Magen;Anastasios Zouzias

  • Affiliations:
  • Department of Computer Science, University of Toronto,;Department of Computer Science, University of Toronto,

  • Venue:
  • APPROX '08 / RANDOM '08 Proceedings of the 11th international workshop, APPROX 2008, and 12th international workshop, RANDOM 2008 on Approximation, Randomization and Combinatorial Optimization: Algorithms and Techniques
  • Year:
  • 2008

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Abstract

Let Pbe a set of npoints in Euclidean space and let 0 茂戮驴Ponto a subspace of dimension $\mathcal{O}(\epsilon^{-2} \log n)$ such that distances change by at most a factor of 1 + 茂戮驴. We consider an extension of this result. Our goal is to find an analogous dimension reduction where not only pairs but all subsets of at most kpoints maintain their volume approximately. More precisely, we require that sets of size s≤ kpreserve their volumes within a factor of (1 + 茂戮驴)s茂戮驴 1. We show that this can be achieved using $\mathcal{O}(\max\{\frac{k}{\epsilon},\epsilon^{-2}\log n\})$ dimensions. This in particular means that for $k = \mathcal{O}(\log n/\epsilon)$ we require no more dimensions (asymptotically) than the special case k= 2, handled by Johnson and Lindenstrauss. Our work improves on a result of Magen (that required as many as $\mathcal{O}(k\epsilon^{-2}\log n)$ dimensions) and is tight up to a factor of $\mathcal{O}(1/\epsilon)$. Another outcome of our work is an alternative and greatly simplified proof of the result of Magen showing that all distances between points and affine subspaces spanned by a small number of points are approximately preserved when projecting onto $\mathcal{O}(k\epsilon^{-2}\log n)$ dimensions.