Covariant Perturbation Theory for Chiral Lagrangians

The formalism "partial differential equations with respect to coupling constants" is developed involving the total Lagrangian density. By means of this formalism the covariant perturbation theory is developed using the example of chiral Lagrangian for massless pions. Because our formalism involves the total Lagrangian density, the covariant perturbation theory easily follows by integrating the partial differential equation with respect to coupling constant for the $S$ matrix. Thus our approach is more direct and simpler than the other approaches involving the interaction Hamiltonian density and which eitheruse use the functional techniques or the generating functionals of the time-ordered Green's functions.