Variational order for forced Lagrangian systems II. Euler–Poincaré equations with forcing

In this paper we provide a variational derivation of the Euler–Poincaré equations for systems subjected to external forces using an adaptation of the techniques introduced by Galley and others Martín de Diego and Martín de Almagro (2018 Nonlinearity 31 3814–3846), Galley (2013 Phys. Rev. Lett. 110 174301), Galley et al (2014 (arXiv:[math-Ph] 1412.3082)). Moreover, we study in detail the underlying geometry which is related to the notion of Poisson groupoid. Finally, we apply the previous construction to the formal derivation of the variational error for numerical integrators of forced Euler–Poincaré equations and the application of this theory to the derivation of geometric integrators for forced systems.