TITLE: Spivak and Invariance of Domain
QUESTION [12 upvotes]: On p.3 of the first volume of Spivak's Comprehensive Introduction to Differential Geometry, he says that it is an "easy exercise" to show that the invariance of domain theorem (if $f:U\subset\mathbb{R}^n\rightarrow\mathbb{R}^n$ is one-to-one and continuous and $U$ is open then $f(U)$ is open) implies that in his definition of a manifold

a metric space $M$ such that every
  point $x\in M$ has a neighborhood $U$
  of $x$ and some integer $n\geq0$ such
  that $U$ is homeomorphic to
  $\mathbb{R}^n$,

the neighborhood $U$ in fact must be open. 
My question: My proof seems to require a bit of set up, as well as two appeals two the invariance of domain theorem, including once to first prove that the dimension of a manifold is well defined (which Spivak discusses on p.4). In any case, I feel like the argument is longer than what Spivak calls a "complicated little argument" on the previous page. Am I missing something obvious?
Perhaps my real question is whether this kind of comment is to be expected from Spivak, since I am reading this book on my own.

REPLY [10 votes]: I also find the "it is an easy exercise" remarks very frustrating. I think (but am not sure) that I came up with a proof, probably similar to yours. This assumes the fact stated on page 2 "we can always choose the neighborhood $U$ in our definition to be an open neighborhood."  I use this fact in choosing $V$ below.
Let $\varphi : U \to \mathbb{R}^n$ be the homeomorphism given in the definition of the Euclidean neighborhood of $x$, and let $z \in U$.
Since $\, z \in M \quad \exists$ an open set $V$ of $M$ with $z \in V$, and a homeomorphism $\theta:V \to \mathbb{R}^n$.
Consider the set $W \equiv V \cap U$.  $W$ is open in the relative topology of $U$, so $\varphi (W)$ is open in $\mathbb{R}^n$. For notational convenience, let $\psi \equiv \varphi \mid_{W}$. The map $\;\theta \circ \psi^{-1} :\varphi (W) \to \mathbb{R}^n$ is continuous, so by Invariance of Domain $\; \theta (W)= \theta \circ \psi^{-1}[\varphi (W)] \;$ is open in $\mathbb{R}^n$, which tells us that $W$ is open in $V$.
Since $V$ is open in $M$, $W$ is open in $M$ and we have $z \in W \subset U$, so $U$ is open.
I wouldn't bet my life that what I did is correct, but I think it is.  This is my first time attempting to answer a question, so if I've messed up, please be gentle.
Thanks,
Dave