On existence of a parameter-sensitive region: quasi-line soliton interactions of the Kadomtsev–Petviashvili I equation

A line-soliton solution can be regarded as the limiting solution with parameters on the boundary between regular and singular regimes in the parameter space of a periodic-soliton solution. We call the periodic soliton with parameters of the neighborhood of the boundary a quasi-line soliton. The solution with parameters on the intersection of the two boundaries, in the parameter space of the two-periodic-soliton solution on which each periodic soliton becomes the line soliton, corresponds to the two-line-soliton solution. On the way of the turning into the two-line-soliton solution from the two-periodic-soliton solution as a parameter point approaches to the intersection, there is a small parameter-sensitive region where the interaction between two quasi-line solitons undergoes a marked change to a small parameter under some conditions. In such a parameter-sensitive region, there is a new long-range interaction between two quasi-line solitons, which seems to be the long-range interaction between two line solitons through the periodic soliton as the messenger. We also show that an attractive interaction between a finite amplitude quasi-line soliton and infinitesimal one is possible.