# The equations of motion for consistency of a post-Newtonian Lagrangian formulation

## TL;DR

This paper introduces a coherent set of equations of motion derived from the Euler-Lagrange equations for post-Newtonian systems, which better conserve constants of motion and are more accurate than traditional truncated equations.

## Contribution

It demonstrates that equations of motion based on generalized momenta provide a coherent, non-truncated alternative to velocity-based equations in post-Newtonian Lagrangian formulations.

## Key findings

- Coherent equations conserve constants of motion more accurately.
- Numerical evaluations confirm high precision with appropriate integrators.
- Differences in dynamical behavior, including chaos, are analyzed.

## Abstract

Equations of motion for a general relativistic post-Newtonian Lagrangian approach mainly refer to acceleration equations, i.e. differential equations of velocities. They are directly from the Euler-Lagrangian equations, and usually have higher-order terms truncated when they remain at the same post-Newtonian order of the Lagrangian. In this sense, they are incoherent equations of the Lagrangian and approximately conserve constants of motion in this system. In this paper, we show that the Euler-Lagrangian equations can also yield the equations of motion for consistency of the Lagrangian in the general case. The coherent equations are the \emph{differential} equations of generalized momenta rather than those of the velocities, and have no terms truncated. The velocities are not integration variables, but they can be solved from the \emph{algebraic} equations of the generalized momenta with an iterative method. Taking weak relativistic fields in the Solar System and strong relativistic fields of compact objects as examples, we numerically evaluate the accuracies of the constants of motion in the two sets of equations of motion. It is confirmed that these accuracies well satisfy the theoretical need if the chosen integrator can provide a high enough precision. The differences in the dynamical behavior of order and chaos between the two sets of equations are also compared. Unlike the incoherent post-Newtonian Lagrangian equations of motion, the coherent ones can theoretically, strictly conserve all integrals in some post-Newtonian Lagrangian problems, and therefore are worth recommending.