# Introduction to Differential Geometry

1. Feb 1, 2006

### JasonRox

So, where do I begin?

I have a current interests in going into Algebraic Topology and I am currently learning about Point-Set Topology and Abstract Algebra, which are pre-requisites to Algebraic Topology.

After talking to a professor, he recommended learning some Differential Geometry, but I think this requires some knowledge of Differential Forms first (not 100% sure).

So can anyone recommend me in a good direction towards Differential Geometry?

What should I know before pursuing this topic? I'm guessing some vector calculus like Green's/Stoke's Theorem and what not.

Any suggestions or advice is greatly appreciated.

Note: I noticed that a lot of Algebraic Topologists also know a lot of Differential Geometry too, which I know is not a coincidence.

2. Feb 1, 2006

### Cexy

It's certainly possible to learn about differential geometry without knowing about differential forms in any detail - I did exactly that for a course on general relativity last term! I'm doing a more advanced course on differential geometry this term, which does involve a lot of differential forms, and it's much nicer.

There are certain things that you should probably be familiar with - obviously proving Stokes' Theorem for differential forms will look pretty weird if you don't know Stokes' Theorem in Euclidean space!

Then there are your basic bread-and-butter preliminaries - functions, vector spaces, linear algebra, groups, tensors, topological spaces, homeomorphisms... I'd say that all of those would be useful.

The book I learnt from was 'Geometry, Topology and Physics' by Nakahara. I thoroughly recommend it - its explanations are clear and it is never vague or hand-wavey. It also has a good introductory section to get you up to speed. It has a fair number of applications to physics, but never too many - and you never know, you might find them interesting!

3. Feb 1, 2006

### JasonRox

You need to know about Groups and Tensors for Differential Geometry/Form?

That seems a little odd.

I might want to add that I obviously want to learn this in rigorous detail. Knowing how to prove Stoke's Theorem is one thing, understanding the proof is another thing.

I had the option to take a General Relativity class with Differential Geometry, and I know that it wasn't going to contain rigorous mathematics. Of course, it was an introduction and I assume yours was too because otherwise Differential Geometry would have been a pre-requisite.

Last edited: Feb 1, 2006
4. Feb 1, 2006

Staff Emeritus
Well, no, as a look into Nakahra will show you. Groups come into principal bundles, which are important in differential geometry, and tensors are an alternative to differential forms - you will eventually have to learn both as they are somewhat complementary. Nakahara is a text for physicists not mathematicians but it should be sufficiently rigorous for your purposes. Or you could ask that professor what text he recommends.

5. Feb 1, 2006

### JasonRox

My professor is actually looking into that I believe.

The problem is that I can't learn something just because it is complementary. Differential Geometry is a complementary to Algebraic Topology, but I don't plan on going into as deep as I'm planning for Algebraic Topology. I just don't have the time to do that. There is no doubt I will know lots of it, but having one topic as broad as Algebraic Topology is already more than a handful.

So, I can't go and learn Tensors just because it is a complementary to one of the topics I learned because that will lead to another complementary and then another and then another.

There is no doubt I will learn some basics about Tensors in some time to come, but I'm concerned about what I need to know that will prepare myself as much as possible for Algebraic Topology.

I'm covering the pre-requisites to Algebraic Topology, but I think doing some Differential Geometry would be beneficial since it will clarify a lot of things with regards to manifolds and what not.

So, I'm just wondering, what are the things that I should know for sure before reading into Differential Geometry?

Try to keep the things reasonable, like topics in Calculus, Linear Algebra, Abstract Algebra and what not. You can take a course in Differential Geometry with certain pre-requisites, what might they be specifically? Certainly it's not something you learn in Graduate School because they offer the course as an Undergraduate Course.

6. Feb 4, 2006

### mathwonk

differential geometry is about curvature, so learn that.

7. Feb 4, 2006

### JasonRox

Can you be more specific?

8. Feb 5, 2006

### mathwonk

well tangent lines, tangent planes, coordinate changes, gradients are all first derivative phenomena.

from a certain point of view, this is all that makes sense on an abstract manifold.

if you add to a manifold the concept of a distance, i.e. a "metric", you can measure angles and curvature, in terms of second derivatives. This subject is what is called differential geometry, i.e. geometry on a manifold that has a metric.

Curvature of curves is measured by comparing with a circle, just as slope is measured by comparing with a line. Gaussian curvature of a surface is calculated by comparing areas of the surface with areas on a sphere.

although defining curvature involves a metric, there is a strong link between intrinsic topology and global properties of curvature. The Gauss Bonnet theorem says the average curvature of a compact surface is governed by the topology of the surface. e.g. neither a sphere nor a torus can have a metric which is everywhere negatively curved.

This link generalizes to the concept of "chern classes" of bundles, intrinsic cohomology classes that can also be computed using curvature, e.g. via "connections".

Or maybe that discussion is less specific. Probably better to just look up "curvature" in the index of various books.

Last edited: Feb 5, 2006
9. Feb 5, 2006

### mathwonk

actually maybe you are more interested in differential, topology than differential geometry. A good place to begin is Spivak's Calculus on manifolds, to learn about manifolds and differential forms. Once you have that language, you might read volume I of his differential geometry book, which really does not have any differential geometry in it, but does have some algebraic and differential topology. Other good sources are Milnor's topology from the differentiable viewpoint (great!) and the differential topology textbook it inspired by Guillemin and Pollack.

10. Feb 6, 2006

### Doodle Bob

Wow, this conversation keeps popping up on this list. Whatever you do, do *not* start with any text written by a Russian or published by Dover or written for physicists. So many people start down that path and are never seen again.

I would recommend that you start with the basics: low-dimensional differential geometry. Millman and Parker's Elements of Differential Geometry and Do Carmo's Differential Geometry of Curves and Surfaces and Oprea's Differential Geometry are all excellent introductions to the field and develop the proper intuition for the subject.

From there, Spivak makes much more sense.

Plus, don't worry about the tensor nonsense. Tensor theory is nothing but a mathematical hobgoblin. Learn about the curvature and the tensors will come on their own.

11. Feb 6, 2006

### JasonRox

I noticed that the Dover editions aren't always straightforward.

I'll take a look into those books. Hopefully our school has them.

12. Feb 6, 2006

### matt grime

Dovers are reprints of Classics. Everyone should look at them but bear in mind that cheap as they are they may be somewhat out of date. Any old book on differential geometry tends to make me want to slit my wrists at the cumbersome notation and presentation. But that is just my recollection from learning it at university, so I apologise if it is warped with the mists of time and distaste for the subject I picked up then

13. Feb 6, 2006

### Doodle Bob

Actually, this isn't quite true. Dovers are reprints of books whose copyrights are cheap enough that they are cheap to print. Thus, a great many of them are old texts that no one quite cares about any more or translated texts whose copyright ownership is problematic at best. Now, there are quite a few good ones out there, but it is just not worth the time and effort to find them, when there are so many much, much, much better and clearer and more contemporary introductions to diffl. geometry such as those I listed.

14. Feb 6, 2006

I did it. It was good. Well, actually, I had seen the version in Euclidian space, but never worked with it and couldn't really remember it. Mostly because things like "Curl" seemed arbitrary and silly. Learning the more general case helped me appreciate the Euclidian version more. In general actually, learning Calculus on Manifolds helped me appreciate the standard multivariable calculus more.

15. Feb 6, 2006

### JasonRox

Yeah, Dovers are fairly old. They are sometimes prints from the 50's.

16. Feb 6, 2006

### mathwonk

i was trying to make the distinction between the language of differential forms and tangent bundles that is widely used in algebraic and differential topology, as opposed to the more specialized concept of curvature which is really the concept peculiar to differential geometry per se.

I.e. I am not convinced anyone interested in algebraic topology as Jason says he is, needs either an elementary or an advanced book on differential geometry, excellent though the ones mentioned by Doodle Bob are.

I assumed he resally wants to learn the language of manifolds, forms, and bundles, as used by algebraic topologists, and that is a different body of material.

For instance it is contained in spivak's "calculus on manifolds" in a very basic undergraduate and elementary form.

then guillemin and pollack use those basic ideas to do intersection theory and transversality.

then spivak's vol 1, treats it more abstractly, but still falls short of using any differential geometry, until volume 2.

Indeed vol2 of spivak is a good introduction to curvature and differential geometry which uses very little of the abstratc machinery of volume 1.

but I think it is vol 1 that Jason wants rather than vol 2. Thus millman and oparker, while a nice easy intro to diff geom, will not give him whart an algebraic topologist needs in my opinion.

I could be wrong of course.

differential geometry is a more specialized subject, and really does need tensors to do it right.

if you want to see the kind of differential tools used in algebraic topology, consult the book of Bott and Tu, Differential Forms in Algebraic Topology (Graduate Texts in Mathematics) (Hardcover)
by Raoul Bott, Loring W. Tu

17. Feb 6, 2006

### JasonRox

That is precisely what I am looking for.

18. Feb 6, 2006

### Doodle Bob

MW,

you might be right on this score. If he's already doing algebraic topology, then he's most likely already dipped into cohomology, which is after all a way to get at forms on manifolds. If this is where Jason wants to go, I would then recommend as well Glen Bredon's fine text, Topology and Geometry.

But what can I say: I'm a Riemannian geometer, so I tend towards the geometry rather than the topology. And I'm getting tired of these endless questions about tensors and forms.

Another plug for yet another fine geometry text: This may be a bit advanced but if anyone wants to get a truly broad spectrum of what differential geometry is all about and where it is going, I highly recommend Marcel Berger's A Panoramic View of Riemannian Geometry. Every time I open this tome, I learn something new. Ans, whether you know much about the topic or not, the first few chapters are quite readable.

19. Feb 6, 2006

### mathwonk

doodlebob, for the intersection between algebraic topology and differential geometry, do you (know or) recommend goldberg's curvature and homology? I hear it is excellent but have never read it.

20. Feb 6, 2006

### Doodle Bob

OK, for my academic grandfather, I'll break my rule: this is a fine text and, yes, it is a Dover book. It motivates quite well the connection between the Riem. Geometry and Algebraic Topology. While I'm on a FISA-level of bad faith, I'll also recommend Bishop and Goldberg's Tensor Analysis on Manifolds.