What is an example of a second order differential equation for which it is known that there are no smooth solutions? I would really appreciate if someone could just write down for me one example of a second order, or higher, differential equation for which it is known that there are no smooth solutions; and it's fine if it's a partial differential equation.
At first I thought it would be easy to either come up with an example or else find one by searching google/wiki/arxiv; but now I am not so sure.
I have a thing for non-smooth functions, and it just bothers me that I don't even know a single example of this type of differential equation.  Thanks!
 A: Consider the partial differential equations associated to the isometric embedding problem of the hyperbolic plane into Euclidean 3-space. In $C^1$ there exists a solution by Nash-Kuiper theorem, but it is known classically that there cannot be solutions that are twice or more continuously differentiable. 
A: How about you take the differential equation
$ \frac{dy}{dx} = |x|  $
This is a linear non-homogeneous differential equation, whose solution is $C^1$ but not smooth at $x=0$.
A: There are already first order linear partial differential equations with smooth coefficients which do not admit any smooth solutions. 
Hans Lewy produced the first example of such a PDE. The equation reads 
$$\left[-i\partial_x+\partial_y-2(x+iy)\partial_z\right]u(x,y,z)=f(x,y,z),\qquad(x,y,z)\in\mathbb R^{3}.$$
The equation does not have distribution solutions in any neighbourhood of any point in $\mathbb R^3$ provided $f=f(x,y,z)$ is not a real analytic function (it can be smooth though).
The original paper by Lewy is nice, clear and less than 4 pages long (freely available here).   
