  The KP-II equation was derived by Kadomtsev and Petviashvili to explain
stability of line solitary waves of shallow water. Using the Darboux
transformations, we study linear stability of 2-line solitons whose line
solitons interact elastically each other. Time evolution of resonant continuous
eigenfunctions is described by a damped wave equation in the transverse
variable which is supposed to be a linear approximation of the local phase
shifts of modulating line solitons.
