I came across this in my textbook. The author says this equation is nonlinear
$$m\frac{{{d^2}}}{{d{t^2}}}x = - {F_0}$$ for x<0 $$m\frac{{{d^2}}}{{d{t^2}}}x = {F_0}$$ for x>0 $$m\frac{{{d^2}}}{{d{t^2}}}x = 0$$for x=0. But as far as I know, the linear differential equation must be a sum of terms each of which contains at most one power of x. It seems like it actually meets this standard. So how can I tell whether one equation is linear or nonlinear?