Preservation of Quadratic Invariants by Semiexplicit Symplectic Integrators for Non-separable Hamiltonian Systems

TL;DR

This paper proves that semiexplicit symplectic integrators for non-separable Hamiltonian systems preserve linear and quadratic invariants, enhancing their structure-preserving properties similar to well-known symplectic methods.

Contribution

It demonstrates that these integrators maintain invariants in non-separable Hamiltonian systems, extending the understanding of their geometric preservation capabilities.

Findings

Preservation of linear invariants in extended phase space

Preservation of quadratic invariants by the integrators

Numerical demonstrations confirming theoretical results

Abstract

We prove that the recently developed semiexplicit symplectic integrators for non-separable Hamiltonian systems preserve any linear and quadratic invariants possessed by the Hamiltonian systems. This is in addition to being symmetric and symplectic as shown in our previous work; hence it shares the crucial structure-preserving properties with some of the well-known symplectic Runge--Kutta methods such as the Gauss--Legendre methods. The proof follows two steps: First we show how the extended Hamiltonian system proposed by Pihajoki inherits linear and quadratic invariants in the extended phase space from the original Hamiltonian system. Then we show that this inheritance in turn implies that our integrator preserves the original linear and quadratic invariants in the original phase space. We also analyze preservation/non-preservation of these invariants by Tao's extended Hamiltonian system and the extended phase space integrators of Pihajoki and Tao. The paper concludes with numerical demonstrations of our results using a simple test case and a system of point vortices.