  The intrachannel interaction of pulses in weakly nonlinear coherent optical
fiber lines is theoretically investigated. It is shown that the main
contribution to the perturbation of the optical field comes from resonant
interactions of ordered triplets of pulses. The structure of triplets is
determined. The weight contributions of such interactions are calculated. A
classification of interactions using Loeschian numbers is proposed. Using
computer simulation, the dependence of the average energy of the optical field
perturbations on the distance is shown. Based on the performed analysis, an
effective algorithm is proposed for assessing the perturbations resulting from
intrachannel interaction.
