# A compact rational Krylov method for large-scale rational eigenvalue problems

## TL;DR

The paper introduces R-CORK, a memory-efficient rational Krylov method for solving large-scale rational eigenvalue problems by leveraging a compact decomposition and a two-level orthogonalization process.

## Contribution

It extends the CORK method to rational eigenvalue problems, enabling efficient solution of large-scale problems with reduced memory and computational costs.

## Key findings

- R-CORK reduces storage and orthogonalization costs compared to classical methods.
- The method is effective for large-scale rational eigenvalue problems.
- Numerical examples demonstrate satisfactory practical performance.

## Abstract

In this work, we propose a new method, termed as R-CORK, for the numerical solution of large-scale rational eigenvalue problems, which is based on a linearization and on a compact decomposition of the rational Krylov subspaces corresponding to this linearization. R-CORK is an extension of the compact rational Krylov method (CORK) introduced very recently by Van Beeumen et al. to solve a family of non-linear eigenvalue problems that can be expressed and linearized in certain particular ways and which include arbitrary polynomial eigenvalue problems, but not arbitrary rational eigenvalue problems. The R-CORK method exploits the structure of the linearized problem by representing the Krylov vectors in a compact form in order to reduce the cost of storage, resulting in a method with two levels of orthogonalization. The first level of orthogonalization works with vectors of the same size that the original problem, and the second level works with vectors of size much smaller than the original problem. Since vectors of the size of the linearization are never stored or orthogonalized, R-CORK is more efficient from the point of views of memory and orthogonalization than the classical rational Krylov method applied directly to the linearization. Taking into account that the R-CORK method is based on a classical rational Krylov method, to implement implicit restarting is also possible and we show how to do it in a memory efficient way. Finally, some numerical examples are included in order to show that the R-CORK method performs satisfactorily in practice.