# Differential Geometery: Images of Gauss Maps

• SNOOTCHIEBOOCHEE
In summary, the question asks about the image curves under the Gauss map of the torus and the saddle surface, and which points of the image are mapped from exactly two points on the surface. For the torus, meridians and parallels map to circles in the xy and xz planes respectively, while the poles of the unit sphere are the only points not mapped from two points on the torus. As for the saddle surface, the Gauss map of a straight line with constant y is requested, as well as the amount of the sphere covered by the entire image of the saddle surface.

## Homework Statement

this is a 2 part problem

let G: T-----> (Sigma) be the gaus map of the torus T derived from its outward unit normal U. What are the image curves under G of the meridians and parallels of T? What points of (Sigma) are the image of exactly two points of T?

And

Let G: M---->(Sigma) be the Gauss map of the saddle surface M: z=xy derived from the unit normal. What is the image under G of one of the straight lines, y= constant in M? How much of the sphere is covered by the entire image G(M)

## Homework Equations

Part 1:

Unit normal of z=xy

U= [-fxU1- fyU2+U3]/ (1+ (fx)^2 + (fy)^2)^1/2

## The Attempt at a Solution

I don't know how to do any rigorous proofs for these, but i am not sure we are suppose to.

Ok for part 1 here is what i think.

I think if you travel along the surface of the torus, along the meridian, you it will be mapped to a circle (in the xy plane). if you travel along the meridians you will get a circle in the xz plane.

And the points in sigma thare are exactly 2 points in T are the North and south poles of the unit sphere.

And for part 2 i am completley lost, am i even on the right track for part one?

any help is appreciated.

can anybody help with this?

You didn't really give us an exact notion of how stuff is oriented, but offhand, I would say meridians of the torus map to meridians of the the sphere. And parallels to parallels. I would also say that poles of the sphere are the only points which are NOT twofold images of points on T. There's an infinite number of points mapping to them. Do you see that picture?