Martins Bruveris François Gay-Balmaz Darryl D. Holm Tudor S. Ratiu The Momentum Map Representation of Images. 115-150 2011 21 J. Nonlinear Sci. 1 https://doi.org/10.1007/s00332-010-9079-5 db/journals/jns/jns21.html#BruverisGHR11