I have a differential equation depending on $z$, $x$ and $t$ in a 2d-system which looks like
$$\partial_z A = \partial_x A+\partial_x^2 A+A+f(t)\cdot A$$
with
$$\frac{df(t)}{dt}=f(A)$$
The formula describes the propagation of light (as the complex function $A$) through material in $z$-direction. Thus I can relate the position of $A$ with the time $t$.
But how can I couple both, so that I can solve the equation? If $f$ is constant, the solution is easy. But when $f$ depends on the time (and thus on the position of the current iteration) I do not know how to proceed.
Is that problem solvable, or do I have to use a numeric approach?