Friday, March 17, 2017

The Unreasonable Effectiveness of Random Orthogonal Embeddings

In the series "The Unreasonable effectiveness of", we've had 

Today, we have something that is a subset of random projections: The Unreasonable Effectiveness of Random Orthogonal Embeddings by Krzysztof Choromanski, Mark Rowland, Adrian Weller
We present a general class of embeddings based on structured random matrices with orthogonal rows which can be applied in many machine learning applications including dimensionality reduction, kernel approximation and locality-sensitive hashing. We show that this class yields improvements over previous state-of-the-art methods either in computational efficiency (while providing similar accuracy) or in accuracy, or both. In particular, we propose the \textit{Orthogonal Johnson-Lindenstrauss Transform} (OJLT) which is as fast as earlier methods yet provably outperforms them in terms of accuracy, leading to a `free lunch' improvement over previous dimensionality reduction mechanisms. We introduce matrices with complex entries that further improve accuracy. Other applications include estimators for certain pointwise nonlinear Gaussian kernels, and speed improvements for approximate nearest-neighbor search in massive datasets with high-dimensional feature vectors.

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