Three-phase solutions of the KP
equation
by B. A. Dubrovin, Ron Flickinger, and Harvey Segur
(as described in Studies in Applied Mathematics, 99,
137-203 (1997))
N-phase solutions of the KP
equation
by Markus Schmies, using the approach of Bobenko and Bordag
(as described in J. Phys. A 22, 1259-1274 (1989))