Variational principles on principal fiber bundles: A geometry theory of Clebsch potentials and Lin constraints

The geometric theory of Lin constraints and variational principles in terms of Clebsch variables proposed recently by Cendra and Marsden [1987] will be generalized to include those systems defined not only on configuration spaces which are products of Lie groups and vector spaces but on configuration spaces which are principal bundles with structural group G. This generalization includes, for example, fluids with free boundaries, Yang-Mills fields, and it will be very useful, as it will be shown later, to illustrate some aspects of the theory of particles moving in a Yang-Mills field in both its variational and Hamiltonian aspects.