Reference request for a proof : every convex open subset of $\Bbb R^n$ is diffeomorphic to $\Bbb R^n$

95 Views Asked by At

Is there a reference of a proof of the following statement?

If $U$ is a nonempty convex open subset of $\Bbb R^n$, then $U$ is diffeomorphic to $\Bbb R^n$.