# How do manifolds have enough structure to do calculus?

I am referring, of course, to to differentiable manifolds. I've seen a few different definitions. The one I like best is the one which says it's a topological space such that every point has a neighborhood homeomorphic to an open set in $\mathbb{R}^n$ and the overlap functions are diffeomorphisms.

I can see that we want the transition maps are diffeomorpisms, so the calculus we do on each coordinate chart agrees with the others, but how do homeomorphisms to Euclidean space give enough structure to develop a derivative, for instance, in a coordinate patch? I mean, the circle and the sphere are homeomorphic but I'd only call one of those smooth. How does smoothness on overlaps do it for the whole manifold? I've seen it said that once a differentiable structure is defined, the homeomorphisms become diffeomorphisms. Is this correct and could somebody expand on that if it is?

Furthermore, I've seen definitions given for manifolds with bijections rather than homeomorphisms. Can that possibly work? Bijections don't have to be particularly nice at all, so I can't for the life of me see how you'd get enough structure out of that. Please correct me if I've misunderstood some part of this.

You don't do calculus directly on the manifold. Everything is done in the world of coordinates.

Your coordinate maps let you push things down into $\mathbb{R}^n$ where we can take limits, compute partials, find Jacobian matrices etc.

Requiring the coordinate maps to be homeomorphisms is redundant. Why? Take your manifold to be a set equipped with some altas (bijective coordinate charts). Then you just declare that these coordinate charts (bijections) are homeomorphisms. In that way you force a topology on the set and turn in into a topological space (now equipped with homeomophism coordinate charts).

The trick to manifold theory is that you're never really working directly on the manifold itself. You always translate to the world of coordinates and work there.

Fair enough, my last paragraph is a bit overstated. Maybe I should say instead...

To do concrete "calculus" computations, you must (usually) translate to the world of coordinates and work there. This is much like it is in geometry. You can do a lot with synthetic geometry, but when you want to calculate...add coordinates and start computing.

• At first most work is done locally but later on one often introduces global objects where there is no need for local computations anymore. – gofvonx Oct 17 '13 at 20:57
• Good point @gofvonx. The same is true of other areas like linear algebra. First you must muck around with coordinates (or matrices) then you get to do nice slick invariant stuff. :) – Bill Cook Oct 17 '13 at 21:00
• This is mainly correct and suffices at first but is a bit overstated in general (the last paragraph). For instance, in differential geometry it is to our advantage in many cases to use coordinate-free formalism of connections, forms and so on. For instance, one first defines smooth functions using coordinates, but then, vectors, vector fields, differentials forms... can (an in many cases are) defined without resorting to coordinates. Eliminating coordinates was Elie Cartan's original motivation for introducing "moving frames", which later lead to the language of connections. – Moishe Kohan Oct 17 '13 at 21:01
• @studiosus Intresting. Do you have any further reading for Cartan's motivation or the historical development in general in this part of differential geometry? – gofvonx Oct 17 '13 at 21:11
• The point of working with a manifold (as opposed to a general topological space or metric space etc.) is that you have coordinates. My answer was particularly aimed at addressing your concern with basic manifold theory (the nuts and bolts of the definition). In this context, you cannot avoid coordinates (without doing something very weird). As you build up the basic results, you get to move away from annoying coordinate dependent arguments. – Bill Cook Oct 17 '13 at 22:03

Given a smooth structure on the manifold $M$ the homeomorphisms become diffeomorphisms. To see that let $\varphi_\alpha:U_\alpha\subset M\to\mathbb{R}^n$ be charts on $M$. A map $f:M\to M$ is said to be smooth, if its local representation $\varphi_\alpha\circ f\circ\varphi_\beta^{-1}:\mathbb{R}^n\to \mathbb{R}^n$ is smooth. Now consider the local representation of a homeomorphism $\varphi_\alpha$ and note that smoothness follows by definition of the smooth structure (since the transition maps $\varphi_\alpha\circ\varphi_\beta^{-1}$ are smooth). Thus, $\varphi_\alpha$ indeed is a diffeomorphism on $M$.

• I've looked at a lot of sources and I don't think I've ever seen smoothness of a map on a manifold defined this way, strangely enough, but it clears a lot up. Thanks. It really annoys me sometimes how much GR textbooks gloss over the interesting differential geometry! It all feels so unmotivated sometimes. Do you have a favorite differential geometry text which covers this stuff? – JohnnyMo1 Oct 17 '13 at 21:59
• @JohnnyMo1 I appreciate John M. Lee's Introduction to Smooth Manifold exactly for the motivation and many exercises which help understanding crucial parts. – gofvonx Oct 17 '13 at 22:08