# Jacobian-free algorithm to calculate the phase sensitivity function in the phase reduction theory and its applications to K\'arm\'an's vortex street

## TL;DR

This paper introduces a Jacobian-free algorithm for efficiently computing the phase sensitivity function in systems with limit cycles, especially applicable to incompressible fluid systems like Kármán's vortex street, reducing computational costs.

## Contribution

The paper presents a novel Jacobian-free numerical method for calculating the phase sensitivity function, enabling applications to complex fluid systems with reduced computational effort.

## Key findings

- The new algorithm significantly reduces computation time.
- Successful application to Kármán's vortex street demonstrates practical utility.
- Method extends phase reduction theory to incompressible fluid systems.

## Abstract

Phase reduction theory has been applied to many systems with limit cycles; however, it has limited applications in incompressible fluid systems. This is because the calculation of the phase sensitivity function, one of the fundamental functions in phase reduction theory, has a high computational cost for systems with a large degree of freedom. Furthermore, incompressible fluid systems have an implicit expression of the Jacobian. To address these issues, we propose a new algorithm to numerically calculate the phase sensitivity function. This algorithm does not require the explicit form of the Jacobian along the limit cycle, and the computational time is significantly reduced, compared with known methods. Along with the description of the method and characteristics, two applications of the method are demonstrated. One application is the traveling pulse in the FitzHugh Nagumo equation in a periodic domain and the other is the K\'arm\'an's vortex street. The response to the perturbation added to the K\'arm\'an's vortex street is discussed in terms of both phase reduction theory and fluid mechanics.