Discrete Lagrangian reduction, discrete Euler-Poincare equations, and semidirect products

Abstract

A discrete version of Lagrangian reduction is developed in the context of discrete time Lagrangian systems on $G\times G$, where $G$ is a Lie group. We consider the case when the Lagrange function is invariant with respect to the action of an isotropy subgroup of a fixed element in the representation space of $G$. In this context the reduction of the discrete Euler-Lagrange equations is shown to lead to the so called discrete Euler-Poincar\'e equations. A constrained variational principle is derived. The Legendre transformation of the discrete Euler-Poincar\'e equations leads to discrete Hamiltonian (Lie-Poisson) systems on a dual space to a semiproduct Lie algebra.