Theorem Egregium: Intrinsic Extrinsic Gauss Curvature of 3D Surfaces

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Discussion Overview

The discussion revolves around the concept of Gauss curvature as defined by Theorem Egregium, particularly focusing on the intrinsic nature of the product of principal curvatures in 3D surfaces and its implications for higher-dimensional hypersurfaces. Participants explore the relationship between principal curvatures and curvature tensors in differential geometry.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that the product of the principal curvatures in Euclidean 3 space is an intrinsic quantity, known as Gauss curvature, as per Theorem Egregium.
  • Another participant questions the relevance of the curvature tensor in the context of the discussion.
  • A different participant clarifies that they are referring to the product of principal curvatures of an embedded hypersurface, which relates to the determinant of the Gauss map.
  • Further elaboration is provided regarding curvature 2-forms in relation to principal directions and the mathematical representation involving principal curvatures and dual forms.

Areas of Agreement / Disagreement

Participants express differing views on the relevance and application of curvature tensors versus the product of principal curvatures, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

There are unresolved assumptions regarding the definitions of curvature in higher dimensions and the specific conditions under which the product of principal curvatures is considered intrinsic or extrinsic.

wofsy
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The product of the principal curvatures of a surface in Euclidean 3 space, though defined extrinsically, is actually an intrinsic quantity, the Gauss curvature. This is the Theorem Egregium.

What about the product of the principal curvatures for higher dimensional hypersurfaces?
 
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Do you read curvature tensor ?
 
Do you read curvature tensor ?

hanskuo I am not sure what your question is. Explain.
 
In difernetial geometry, curvature is a tensor in higher dimensions.(more than 3 dimensions)
This is what I said curvature tensor.
 
I am not referring to the curvature tensor but to the product of the principal curvatures of an embedded hypersurface. This product is the same as the determinant of the Gauss map.
 
hanskuo, I apologize. I guess I lied a little. If one has already found principal directions on a hypersurface then the curvature 2-forms with respect to a principal frame field are the pairwise products of the principal curvatures times the wedge products of the dual 1-forms. In tensor language, if Ei are the principal directions then the curvature 2 forms are
R(X,Y,Ei,Ej) and the relevant equation is

R(X,Y,Ei,Ej) = KiKjEi*^Ej* where Ki is the i'th principal curvature and Ei* is the i'th dual i-form.
 

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