# Stochastic Lie group integrators

## TL;DR

This paper introduces stochastic Lie group integrators that preserve manifold structure for nonlinear stochastic differential equations, combining Lie group actions, exponential maps, and stochastic exponential Lie series for improved accuracy.

## Contribution

It develops stochastic Munthe-Kaas and Castell--Gaines methods for Lie group integration of stochastic differential equations, ensuring solutions stay on the manifold and outperform stochastic Taylor schemes.

## Key findings

- Methods accurately simulate stochastic rigid body dynamics.
- Stochastic Castell--Gaines methods are more accurate than stochastic Taylor schemes.
- Proposed schemes preserve geometric structure in stochastic settings.

## Abstract

We present Lie group integrators for nonlinear stochastic differential equations with non-commutative vector fields whose solution evolves on a smooth finite dimensional manifold. Given a Lie group action that generates transport along the manifold, we pull back the stochastic flow on the manifold to the Lie group via the action, and subsequently pull back the flow to the corresponding Lie algebra via the exponential map. We construct an approximation to the stochastic flow in the Lie algebra via closed operations and then push back to the Lie group and then to the manifold, thus ensuring our approximation lies in the manifold. We call such schemes stochastic Munthe-Kaas methods after their deterministic counterparts. We also present stochastic Lie group integration schemes based on Castell--Gaines methods. These involve using an underlying ordinary differential integrator to approximate the flow generated by a truncated stochastic exponential Lie series. They become stochastic Lie group integrator schemes if we use Munthe-Kaas methods as the underlying ordinary differential integrator. Further, we show that some Castell--Gaines methods are uniformly more accurate than the corresponding stochastic Taylor schemes. Lastly we demonstrate our methods by simulating the dynamics of a free rigid body such as a satellite and an autonomous underwater vehicle both perturbed by two independent multiplicative stochastic noise processes.