Vasilis Margonis

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2ranked-venue papers
0as first author
1since 2021 · last 2023
—ORCID · none

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Theory of computation · 2 · 1 since 2021
YearPublicationVenuePosition
2023 Near-neighbor preserving dimension reduction via coverings for doubling subsets of ℓ1
Ioannis Z. Emiris, Vasilis Margonis, Ioannis Psarros
Theor. Comput. Sci.2
2019 Near-Neighbor Preserving Dimension Reduction for Doubling Subsets of l1
abstract
Randomized dimensionality reduction has been recognized as one of the fundamental techniques in handling high-dimensional data. Starting with the celebrated Johnson-Lindenstrauss Lemma, such reductions have been studied in depth for the Euclidean (l_2) metric, but much less for the Manhattan (l_1) metric. Our primary motivation is the approximate nearest neighbor problem in l_1. We exploit its reduction to the decision-with-witness version, called approximate near neighbor, which incurs a roughly logarithmic overhead. In 2007, Indyk and Naor, in the context of approximate nearest neighbors, introduced the notion of nearest neighbor-preserving embeddings. These are randomized embeddings between two metric spaces with guaranteed bounded distortion only for the distances between a query point and a point set. Such embeddings are known to exist for both l_2 and l_1 metrics, as well as for doubling subsets of l_2. The case that remained open were doubling subsets of l_1. In this paper, we propose a dimension reduction by means of a near neighbor-preserving embedding for doubling subsets of l_1. Our approach is to represent the pointset with a carefully chosen covering set, then randomly project the latter. We study two types of covering sets: c-approximate r-nets and randomly shifted grids, and we discuss the tradeoff between them in terms of preprocessing time and target dimension. We employ Cauchy variables: certain concentration bounds derived should be of independent interest.
Ioannis Z. Emiris, Vasilis Margonis, Ioannis Psarros
APPROX-RANDOM2