
#1
Feb1612, 10:26 AM

P: 206

Hi,
I know that the mean curvature at an extremum point where the function vanishes must be nonpositive.can this say someting about the sign of the mean curvature at the farthest point on a close surface from the origin? Thank's Hedi 



#2
Feb1712, 07:43 AM

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#3
Feb1712, 09:25 AM

P: 206

I mean a real smooth function of two variables whose graph is a closed surface in R3




#4
Feb1712, 10:40 AM

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mean curvatureIsn't the mean curvature of the standard sphere strictly positive  in fact for any surface of positive Gauss curvature? There is a therem that says that any closed surface in 3 space must have a point of positive Gauss curvature. At this point the mean curvature is positive. 



#5
Feb1712, 01:43 PM

P: 206

But Gauss curvature is positive also if the two principal curvatures are negative.at the farthest point from the origin, the surface is enclosed within a ball whose boundary shares a common tangent plane with the surface at this point.It seems that this implies that the mean curvature of the surface at this point must have a definite sign.I don't know what sign and how to explain it.




#6
Feb1712, 01:47 PM

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I am having trouble visualizing the case of both negative principal curvatures. May it can happen at a single point but in a region?can you give an example? 



#7
Feb1712, 01:59 PM

P: 206

The surface must be convex at the farthest point so if this forces positive mean corvature'we are done,but i am not sure of it.




#8
Feb1712, 03:51 PM

P: 206

At a saddle point Gauss curvature is negative so one of the principal directions is negative.this may help for imagining a negative principal direction.




#9
Feb1712, 06:38 PM

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P: 1,716

A classical argument says  surround the surface with a very large sphere centered at the origin and let it's radius shrink until it first touches the surface.At this point the surface and the sphere are tangent and the entire surface lies on the inside of the sphere. Therefore the surface must be convex at this point. 



#10
Feb1812, 10:42 AM

P: 206

Right.Doed it implies something about the sign of the mean curvature at the farthest point?




#11
Feb1812, 01:29 PM

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