Equivalent Lagrangian densities and invariant collective coordinates equations

In the framework of the Lagrangian formalism the partial differential equation under study does not define univocally the Lagrangian density. In this paper we obtain a necessary and sufficient consistency condition over the ansatz that assures the invariance of the collective coordinates (CCs) equations under the change of equivalent Lagrangian densities. When this condition is not fulfilled we show explicitly that different equations of CCs may emerge from equivalent Lagrangian densities and a good agreement between the CCs and the partial differential equations is not expected.