For the Inverse function theorem, the theorem proved the existence of a inverse relation on a local scale, that is if $Df(x)$ is invertible, $f$ is $C^1$ function and $f$: open set E $\subset R^2$ -> $R^2$ , then there exists an open ball U $\subset E$, such that $f(U) $ is also open, and it is a 1-1 relation.
I am wondering that if I want to prove that a function is 1-1 in its whole domain, can I use Inverse function theorem to prove.
Best Answer
Global invertibility theorems are generally hard. One example of such a theorem is stated in Global invertibility of a map $\mathbb{R}^n\to \mathbb{R}^n$ from everywhere local invertibility, where pointers to literature are given. Proofs tend to involve substantially more topology than the local consideration: e.g., topological degree of a map, path lifting...
Here's a simple result of purely analytic nature: if $f$ is differentiable in a convex domain $U\subset \mathbb R^n$ and $\|Df - I\|<1$ pointwise, then $f$ is invertible. (Here $I$ is the identity matrix, and the norm is the operator norm.) Indeed, let $g(x)=f(x)-x$ and observe that $\|Dg\|<1$, hence $|g(a)- g(b)|<|a-b|$ whenever $a\ne b$. It follows that $f(a)\ne f(b)$.
Here is more interesting result. Suppose that $U$ and $V$ are Jordan domains in the plane, $f: \overline{U}\to \overline{V}$ is continuous, the restriction of $f$ to $U$ is differentiable with nonvanishing Jacobian determinant, and the restriction of $f$ to $\partial U$ is a homeomorphism onto $\partial V$. Then $f$ is a diffeomorphism of $U$ onto $V$.
The proof goes as following:
The proof works in higher dimensions too, and the assumption on the boundary values can be weakened.
Unfortunately, I don't know of a good source for this material in finite-dimensional setting; it seems that people interested in global invertibility tend to work in nonlinear functional analysis. I can recommend the book Nonlinear Functional Analysis by K. Deimling: its first chapter is on finite dimensional spaces, and global invertibility is considered briefly in Chapter 4.