Lump type solutions: Backlund transformation and spectral properties

There are various different ways to obtain traveling waves of lump type for the KP equation. We propose a general and simple approach to derive them via a Backlund transformation. This enables us to establish an explicit connection between those low energy solutions and high energy ones. Based on this construction, spectral analysis of the degree $6$ solutions is then carried out in details. The analysis of higher energy ones can be done in an inductive way.