In light-cone coordinates, the sine-Gordon equation can be written . Its inverse scattering transform, solitons, breathers, and scaling symmetries make it an integrable system.
The transformationsform a one-parameter Lie point symmetry of . The product is a similarity variable, and a group-invariant solution satisfies
A sine-Gordon breather is a spatially localized solution that is periodic in time and can be interpreted through a complex-conjugate pair of discrete scattering eigenvalues.