Low Redundancy Estimation of Correlation Matrices for Time Series Using Triangular Bounds.

The dramatic increase in the availability of large collections of time series requires new approaches for scalable time series analysis. Correlation analysis for all pairs of time series is a fundamental first step of analysis of such data but is particularly hard for large collections of time series due to its quadratic complexity. State-of-the-art approaches focus on efficiently approximating correlations larger than a hard threshold or compressing fully computed correlation matrices in hindsight. In contrast, we aim at estimates for the full pairwise correlation structure without computing and storing all pairwise correlations. We introduce the novel problem of low redundancy estimation for correlation matrices to capture the complete correlation structure with as few parameters and correlation computations as possible. We propose a novel estimation algorithm that is very efficient and comes with formal approximation guarantees. Our algorithm avoids the computation of redundant blocks in the correlation matrix to drastically reduce time and space complexity of estimation. We perform an extensive empirical evaluation of our approach and show that we obtain high-quality estimates with drastically reduced space requirements on a large variety of datasets.