Routhian mechanics

Summary

Routhian mechanics is a mixed formulation of mechanics that treats some coordinates in the Lagrangian way and the others in the Hamiltonian way. It is used mainly when some coordinates are cyclic, so that their momenta are conserved. The Routhian replaces the Lagrangian, the conserved momenta become constants, and the problem reduces to fewer degrees of freedom. It combines ideas of Lagrangian mechanics and Hamiltonian mechanics. See the definition.