# Acceleration of the Power Method with Dynamic Mode Decomposition

## TL;DR

This paper introduces a DMD-accelerated power method that significantly reduces iterations needed to find dominant eigenmodes, especially useful as a post-processing step when traditional eigensolvers are unsuitable.

## Contribution

The paper presents a novel DMD-based acceleration technique for the power method, enabling faster convergence and applicability as a post-process in existing power-method workflows.

## Key findings

- DMD-PM(n) reduces power iterations by about 5 times.
- Arnoldi's method outperforms DMD-PM(n) in efficiency.
- DMD-PM(n) is useful as a post-processing tool for power methods.

## Abstract

Presented is an algorithm based on dynamic mode decomposition (DMD) for acceleration of the power method (PM). The power method is a simple technique for determining the dominant eigenmode of an operator $\mathbf{A}$, and variants of the power method are widely used in reactor analysis. Dynamic mode decomposition is an algorithm for decomposing a time-series of spatially-dependent data and producing an explicit-in-time reconstruction for that data. By viewing successive power-method iterates as snapshots of a time-varying system tending toward a steady state, DMD can be used to predict that steady state using (a sometime surprisingly small) $n$ iterates. The process of generating snapshots with the power method and extrapolating forward with DMD can be repeated. The resulting restarted, DMD-accelerated power method (or DMD-PM($n$)) was applied to the two-dimensional IAEA diffusion benchmark and compared to the unaccelerated power method and Arnoldi's method. Results indicate that DMD-PM($n$) can reduce the number of power iterations required by approximately a factor of 5. However, Arnoldi's method always outperformed DMD-PM($n$) for an equivalent number of matrix-vector products $\mathbf{Av}$. In other words, DMD-PM($n$) cannot compete with leading eigensolvers if one is not limited to snapshots produced by the power method. Contrarily, DMD-PM($n$) can be readily applied as a post process to existing power-method applications for which Arnoldi and similar methods are not directly applicable. A slight variation of the method was also found to produce reasonable approximations to the first and second harmonics without substantially affecting convergence of the dominant mode.