Department of Computer Science | Institute of Theoretical Computer Science | CADMO

Prof. Emo Welzl and Prof. Bernd Gärtner

Mittagsseminar Talk Information |

**Date and Time**: Tuesday, May 02, 2017, 12:15 pm

**Duration**: 30 minutes

**Location**: CAB G51

**Speaker**: Kenneth Clarkson (IBM Research Almaden)

A number of matrices that arise in machine learning and data analysis are symmetric positive semidefinite (PSD), including covariance matrices, kernel matrices, Laplacian matrices, random dot product graph models, and others. A common task related to such matrices is to approximate them with a low-rank matrix, for efficiency or statistical inference; spectral clustering, kernel PCA, manifold learning, and Gaussian process regression can all involve this task. Given a square n by n matrix A, target rank k, and error parameter epsilon, we show how to find a PSD matrix B of rank k, such that the Frobenius norm of A-B is within (1+epsilon) of best possible; our algorithm needs O(nnz(A) + n*poly(k/epsilon)) time to do this, where nnz(A) is the number of nonzero entries of A. We also show how to find such a rank-k matrix PSD B of the form CUC^T, where the O(k/eps) columns of C are a subset of those of A, with a similar runtime. Joint work with David Woodruff.

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