Like in heat equation $U_{t} + U_{xx} + bU_{x} + U = 0$. Just let $v(t,x) = e^{t}u(t,x+bt)$, then we turn the problem into a standard one.
Now the question is how to turn the wave equation $U_{tt} - U_{xx} +aU_{t}+ bU_{x} + U = 0$ into standard form by similar trick?
Thanks
