Sine-Gordon equation

The sine-Gordon equation is a nonlinear partial differential equation of the form: \[ \frac{\partial^2 \phi}{\partial t^2} - \frac{\partial^2 \phi}{\partial x^2} + \sin(\phi) = 0 \] where \(\phi\) is a function of two variables, time \(t\) and spatial coordinate \(x\).