Sampling from large matrices: An approach through geometric functional analysis

Reviewer: Bruce E. Litow

This paper explores randomized sampling of matrices by submatrices. This is not a new topic, but the method employed is new and interesting. The main results are somewhat involved, but the key ideas used throughout the paper are these (where A is a finite dimensional matrix, not necessarily square): (1) Use of numerical rank, which exhibits stability not shared by the rank. This is defined as: where the numerator has the Frobenius norm, which is the sum of the squares of the singular values of A, and the denominator has the ℓ 2 norm, that is, the maximum singular value. The sampling parameter (number of rows needed) is bounded above by O(r ·log r). (See Theorem 1.1 of the paper.) The O-notation hides 1η 4·Δ, where 0 < η,Δ < 1, 1 - 2exp(-O(1Δ)) is the probability of sampling success (O notation here indicates an absolute constant), and η determines the error. (2) A law of large numbers for operator-valued random variables. This is the central contribution of the paper and represents an approach distinct from linear algebra techniques. This also allows for a natural notion of row or column sampling of A. (3) A series of tail distribution bonds on expected values of sequences of vectors. These results can undoubtedly be used in areas not covered in this paper, and so assume independent interest. Although the paper is not self-contained, the citations are sufficient for further exploration and the presentation is crisp and quite clear. Online Computing Reviews Service