Linear First Order PDE- Looking for an example

Hi All,

I have an examinable question that I am stuck on and obviously I cannot submit it here.

Instead I am asking if some one could provide a simple problem that demonstrates the principle. Here goes:

We have linear first order PDE u(x,y). Asked to find u given intial conditions which I can.

Next it asks to show that the solution is not defined when y > f(x). (I know what the f(x) of x is but cannot show it)

Could anyone provide a simple example to demonstrate this or at least what do i do?

Thanks

bugatti79