## Surfaces (Pt. I)

**Point of Post: **In this post we start discussing surfaces, giving their motivation, as well as their formal definition.

*Motivation*

In this post we finally begin discussing the real meat, the cool part of geometry (at least the narrow part of it I am currently learning). In particular, in this post we start discussing *surfaces*. I imagine everyone who would read this understands, at least from an intuitive non-mathematically minded sense, what a surface is. This is simultaneously a fantastic and horrible situation. The fact that every average Joe and Janet knows what a surface is tells one that they are, if nothing else, prevalent in the real world, and in particular, the academic sphere. Also, this familiarity with the idea of a surface often gives one intuition about what precisely is happening, or more often then not, what precisely *should *happen. That said, having an idea about what a surface is means that when the, somewhat involved, definition of a surface is given, people are upset that isn’t as ‘simple’ as they’d hoped. Moreover, this preconceived notions often times leads to false intuition, beliefs that are wildly false.

Regardless, what is a surface, and why are they important? Surfaces can be, and almost always are, best described as ‘locally -Euclidean’ spaces. Of course, this just transfers the intuitive idea of surface to the seemingly complicated idea of ‘locally -Euclidean’. Luckily though, although scary sounding, being locally -Euclidean is a very concrete and visually appealing idea. Allow me some license, and imagine that one had a gift wrapped Christmas present sitting in front of them right now. If asked “what is the dimensionality of the wrapping paper enveloping the present” one is apt to say “Oh, well, clearly the wrapping paper is a three-dimensional object!” Indeed, the wrapping paper ‘takes up three dimensions’ in the sense that it extends in all three spacial dimensions, etc. But, this was only after the work of actually wrapping the present. For example, suppose that this was a very large present, and it had taken several sheets of wrapping paper, taped together, to form the wrapping paper cover of the present. So, while the entirety of the taped-together wrapping paper cover is a ‘three dimensional object’ it is comprised of several sheets of wrapping paper which are (being just ‘planes’) two-dimensional objects. Another way to say this is that each point on the wrapping paper cover lives on one of the individual wrapping paper sheets, and so to the point, it might as well be living in two-dimensional space since all it ‘sees’ is the two dimensional sheet it inhabits. This is precisely what locally -Euclidean seeks to capture in its definition. Namely, a space is locally -Euclidean if it can be ‘built’ out of -dimensional objects, or more precisely, if around each point the space ‘looks like’ a -dimensional object. The classic example put into the discussion of surfaces is the surface of the Earth. No one would dispute that the surface of the earth is a three-dimensional object, but to each of us (the ‘points’) what we see are flat floors and straight line horizons. In other words, to us the Earth looks flat, it looks like the plane .

So, why are surfaces important? Besides the obvious answer that surfaces are all around us, they are the spaces on which we operate, on which we live, there are more mathematically interesting. Surfaces encompass two of the most interesting kinds of geometric objects: common loci of sets of functions and graphs. To be more explicit, given a sufficiently nice mapping and sufficiently nice points the level set will be surfaces. Moreover, if is a sufficiently nice function then the graph will be a surface. These are two fundamental objects associated to functions and . Indeed, the rigorous notion of a surface has its origins (although certainly not exclusively) upon the considerations of such objects (graphs and loci) when studying the functions themselves.

Something that was omitted in the above description, hidden implicitly in terms such as ‘sufficiently nice’, is that we for us, for the kind of geometry we want to do, plain old locally -Euclidean spaces aren’t going to be good enough. Indeed, what we’d like to do on surfaces is do calculus. We’d like to define notions of differentiability and integrability, etc. so that we can study the geometric properties of surfaces via analytic methods. Consequently, certain types of surfaces which evidently satisfy the locally -Euclidean (such as the wrapping paper example (depending on precisely what was meant by the wrapping paper)). So, how precisely do we hope to do calculus on surfaces? Well, at least how we plan to do differential calculus should be intuitive enough. Namely, since differential calculus is a local theory we should be able to appeal to the local Euclideaness of our spaces to do calculus much the same as we would do in .

*Topological and Smooth Surfaces*

We now make precise what we mean by locally -Euclidean. Namely, we call a subset of a *topological surface *if for every there exists a neighborhood (in with the subspace topology) of which is homeomorphic to an open subset of . If is such a homeomorphism (with open) we call the ordered pair a *topological chart at *or just a *topological chart *when the base point isn’t important*.* A collection of charts on such that is called an *topological atlas *for . We see then that a subset of is a topological surface if and only if it admits an atlas. We define the *transition map * between the intersecting charts and to be the map given by .

With this in mind, we can now define the objects of our future study. A subset of is called a *smooth regular surface *or just *surface* if it admits a topological atlas such that for every the map is smooth and the total derivative is injective for each . Such a topological atlas is called a *smooth regular atlas *or just *atlas* for short.

We note that since the Jacobian matrix (where, as usual if then where denotes the partial derivative) that being injective is equivalent to and being linearly independent for each . To beat a dead horse, we emphasize for computational purposes that being injective is equivalent to being non-zero, where denotes the usual cross product.

I feel as though at this point some examples are in order. I submit for your approval the absolutely classic example (the abstractification of the surface of the earth example):

* Example: *Let denote the usual -sphere given by . Then, is a surface. Indeed, for each we let . Clearly then is open in . Let be some linear isomorphism sending . Consider then the mapping , where is the unit disc in , given by . Then, is easily seen to be a smooth bijection. Noting then that

It’s easy to see that the columns of the inner matrix are linearly independent and so is injective. Thus, we may conclude that is an atlas for and so is a surface.

We submit our second example as more of a theorem:

**Theorem: ***Let , where is open, be a smooth function. Then, the graph is a surface.*

**Proof: **Define . Clearly is a smooth and bijective, with continuous inverse . We claim now that is injective for each . To see this we merely note that and and clearly these two are linearly independent, from where it follows that is a smooth chart. But, since we see that is an atlas for from where the conclusion follows.

**References:**

[1] Carmo, Manfredo Perdigão Do. *Differential Geometry of Curves and Surfaces*. Upper Saddle River, NJ: Prentice-Hall, 1976. Print.

[2] Pressley, Andrew. *Elementary Differential Geometry*. London: Springer, 2001. Print.

[3] Montiel, Sebastián, A. Ros, and Donald G. Babbitt. *Curves and Surfaces*. Providence, RI: American Mathematical Society, 2009. Print.

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