Riemannian Manifolds and Completeness

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SUMMARY

The discussion centers on the relationship between smooth Riemannian metrics and the completeness of manifolds. It establishes that if every smooth Riemannian metric on a manifold M is complete, then M must be compact. The user attempts to solve the problem by demonstrating that M is a closed manifold and references the Hopf-Rinow theorem, which states that a complete Riemannian manifold is both geodesically complete and totally bounded. The user seeks guidance on how to leverage the invariance of completeness to show boundedness.

PREREQUISITES
  • Understanding of Riemannian geometry and smooth manifolds
  • Familiarity with the Hopf-Rinow theorem
  • Knowledge of the concepts of completeness and compactness in topology
  • Basic principles of metric spaces and boundedness
NEXT STEPS
  • Study the Hopf-Rinow theorem in detail to understand its implications for Riemannian manifolds
  • Research the concept of total boundedness in the context of metric spaces
  • Explore the invariance of completeness under different metrics
  • Investigate examples of compact Riemannian manifolds to solidify understanding
USEFUL FOR

Mathematicians, particularly those specializing in differential geometry, students studying Riemannian manifolds, and anyone interested in the properties of completeness and compactness in topology.

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Homework Statement


Suppose that for every smooth Riemannian metric on a manifold M, M is complete. Show that M is compact.

2. The attempt at a solution

I'm honestly not too sure how to start this question. If we could show that the manifold is totally bounded we would be done, but I'm not sure how to get that out of the assumption. Any ideas that I could play around with?
 
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So I showed that M must be a closed manifold. By Hopf-Rinow if I can show it's bounded then I'll be done. I didn't use the invariance of completeness under arbitrary metrics in my closed argument so I think it will come in use for the bounded part.
 

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