where μ is a spacetime index, ∂ ∂ 0 = ∂ ∂ / ∂ ∂ t , ∂ ∂ 1 = ∂ ∂ / ∂ ∂ x 1 , etc. The summation over the index μ has been omitted following the Einstein notation. If the parameter λ is sufficiently small, then the interacting theory described by the above Lagrangian can be considered as a small perturbation from the free theory.