Tensor rank reduction via coordinate flows

Recently, there has been a growing interest in efficient numerical algorithms based on tensor networks and low-rank techniques to approximate high-dimensional functions and the numerical solution to high-dimensional PDEs. In this paper, we propose a new tensor rank reduction method that leverages coordinate flows and can greatly increase the efficiency of high-dimensional tensor approximation algorithms. The idea is very simple: given a multivariate function, determine a coordinate transformation so that the function in the new coordinate system has smaller tensor rank. We restrict our analysis to linear coordinate transformations, which give rise to a new class of functions that we refer to as tensor ridge functions. By leveraging coordinate flows and tensor ridge functions, we develop an optimization method based on Riemannian gradient descent for determining a quasi-optimal linear coordinate transformation for tensor rank reduction. The theoretical results we present for rank reduction via linear coordinate transformations can be generalized to larger classes of nonlinear transformations. We demonstrate the effectiveness of the proposed new tensor rank reduction method on prototype function approximation problems, and in computing the numerical solution of the Liouville equation in dimensions three and five.