Static solitons, Lorentz invariance, and a new perspective on the   integrability of the Sine Gordon equation in (1+2) dimensions
  Contrary to the common understanding, the Sine-Gordon equation in (1+2) dimensions does have N-soliton solutions for any N. The Hirota algorithm allows for the construction of static N-soliton solutions (i.e., solutions that do not depend on time) of that equation for any N. Lorentz transforming the static solutions yields N-soliton solutions in any moving frame. They are scalar functions under Lorentz transformations. In an N-soliton solution in a moving frame, (N-2) of the (1+2)-dimensional momentum vectors of the solitons are linear combinations of the two remaining vectors. 
Yair Zarmi
nlin.SI