k+ = 0 modes in light-cone quantization

We investigate the light-cone quantization of φ³-theory in (1 + 1) dimensions with a regularization of the discretized light-cone momentum k⁺. Solving a second-class constraint associated with the k⁺ = 0 mode, we show that the k⁺ = 0 mode propagates along the internal lines of Feynman diagrams in any order of perturbation, hence our theory recovers Lorentz invariance.