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A surface is a two-dimensional topological manifold. The term can also be used to describe a smooth surface, depending on the context.

10 votes

Tangent fields spanning the distribution of principal directions on a surface

No. Consider the case of an ellipsoid with three distinct axes, and remove the four umbilic points. Then you cannot find such vector fields on a (punctured) neighborhood of the deleted umbilics. Ha …
Robert Bryant's user avatar
18 votes
Accepted

Flat metric on compact surface minus a point

By the classification of surfaces, the above two cases cover all of the compact, connected surfaces $M$ with $\chi(M)\le 0$. …
Robert Bryant's user avatar
13 votes

generalisation of umbilic surfaces

In general, surfaces in $\mathbb{E}^3$ for which the principal curvatures satisfy a given functional relation $F(\kappa_1,\kappa_2)=0$ are said to be Weingarten surfaces (of type $F$), and the condition … The general theory tells you that, locally, the Weingarten surfaces of type $F$ depend on 2 'arbitrary' functions of one variable. In particular, there are lots of such surfaces locally. …
Robert Bryant's user avatar
8 votes
Accepted

homogeneous surface in $\mathbb{R}^4$

Thus, homogeneous negatively curved surfaces do not exist in $\mathbb{R}^4$. … In higher dimensions, of course, the number of types of homogeneous surfaces increases. …
Robert Bryant's user avatar
18 votes
Accepted

When does the shape operator commute with a derivative?

I wrote an article about this problem, On surfaces with prescribed shape operator, Results in Mathematics 40 (2001), 88–121, that describes what was known about it in 2001 and gives references to the literature …
Robert Bryant's user avatar
4 votes
Accepted

Surface Laplace-Beltrami without coordinates, exterior calculus?

You probably will disallow this, but the following recipe does work: First, let $\nabla\phi:M\to\mathbb{R}^3$ be the (unique) vector-valued function that satisfies $$ d\phi(X) = \nabla\phi\cdot df(X) …
Robert Bryant's user avatar
13 votes
Accepted

Which surfaces have only a finite number of connecting geodesics?

Any smooth compact surface smoothly embedded in $\mathbb{R}^3$ that is not the $2$-sphere must have an infinite fundamental group and hence must have infinitely many distinct (in your sense) geodesics …
Robert Bryant's user avatar
17 votes
Accepted

Geodesics on the twisted pseudosphere (Dini's surface)

To answer Joseph's questions: First, it's not impossible to integrate the geodesic flow of the hyperbolic plane in these coordinates, but the formulae I got aren't very nice, so I'm not going to type …
Robert Bryant's user avatar
8 votes

Intrinsic vs Extrinsic geometry of convex surfaces

But the existence of an umbilic point on the sphere follows from topological considerations: The sum of the Hopf indices of the umbilics is 1 (by a theorem of Hopf) so there has to be at least one umb …
Robert Bryant's user avatar
8 votes
Accepted

Building a geodesic conjugate parameterization on catenoid

I thought about your problem and realized that there is no coordinate parametrization of the catenoid with the properties that you want. Here is my argument: First, note that, in the given $uv$-param …
Robert Bryant's user avatar