We can apply the method of characteristics to solve the equation Pi + c(x, t)px = Owith initial data p(x, 0) = Po(x). We define characteristic curves such as along each of them, p(x, 1) is a function depending on x and 7. O Plx.t) is a function depending on e(x, 1). P(x.) is a function of t only but not a constant. plx.1) is a constant.