“…In the presence of a symmetry by the (left) action of G, we arrive at a reduced Lagrangian function(al) L which is free from the group variable. In this case, the first term in the right hand side of (1.1) drops, and the dynamics is governed by the Euler-Poincaré equations (1.2) d dt δL δξ = −ad * ξ δL δξ , on the Lie algebra g. The Euler-Poincaré equation has been written for a wide spectrum of Lie groups; from matrix Lie groups to diffeomorphism groups, see for instance [26,37,15,16,23,4,24] and the references therein. It is also possible to find various different forms of the Euler-Poincaré equations in the literature.…”