# What is the intrinsic ( without any reference to embedding ) and coordinate free , basis free definition of a 2 sphere as a differentiable manifold?

The surface of such an object would be $$\dfrac{C^2}\pi$$, where $$C$$ is the circumference, which should be derivable from the definition.

• If defined as a Riemann manifold, what would it's metric be? (A smoothly varying inner product of tangent vectors)
• What would other defining properties, such as Gaussian curvature, be when described in a coordinate-free intrinsic way? (Without radii of curvatures or local coordinate charts)

Edit: Further clarification : A metric is just a section of the tensor product of the cotangent bundle with itself . It is a geometric object. I am asking for a geometric definition of the manifold and thus its metric.. not a representation in coordinates or some other made up structure. The question is what is the intrinsic , invariant definition of the sphere as a manifold I am new to differential geometry and I woul like to learn it properly...

• For future posts, using mathjax will make your questions much more readable and, in turn, solicit more answers. – Victoria M Feb 10 at 17:12
• Welcome the Mathematics Stack Exchange community. A quick tour of the site will help you get the most of your time here. For typesetting your equations, please use MathJax. Here is a great reference. – dantopa Feb 10 at 17:13
• The title of the question refers just to a differentiable manifold structure of a sphere whereas the content of the question refers to a particular Riemannian metric on the sphere, which is an additional structure. In particular the $2$-sphere (the differentiable manifold) admits many distinct (nonisometric) Riemannian metrics. – Travis Feb 10 at 21:12
• @Schaurberger Stop insulting people. – Michael Greinecker Feb 11 at 8:53

## 1 Answer

Here are two possible definitions of the round metric on the 2-sphere of radius 1.

Using polar coordinates (strictly speaking not defined at the poles) $$(\theta,\phi)\in[0,\phi]\times [0,2\pi)$$ $$\begin{equation} g = \mathrm{d}\theta^2 +\sin^2\theta \, \mathrm{d}\phi^2 \end{equation}$$

Pullback with respect to the inclusion $$\iota:S^2\hookrightarrow\mathbb{R}^3$$ of the Euclidean metric $$g_E$$ $$\begin{equation} g=\iota^* g_E, \quad g_E=\mathrm{d}x^2+\mathrm{d}y^2+\mathrm{d}z^2. \end{equation}$$

Volume form, volume (up to a sign until you specify an orientation) curvature and so forth can all be derived from the metric. But these are are the topics of a basic course in differential/Riemannian geometry and there is no much point in summarising them here. You should probably find a book (there are many threads on this) and read about it if you are interested.