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The Morse critical-point sum is the Euler characteristic
Statement
Assume the Axiom of Choice (The Axiom of Choice). Let be a closed smooth -manifold with , and let be a Morse function (Morse functions and excellent Morse functions) and let be a Riemannian metric on (Every smooth manifold admits a riemannian metric). Then the sum over the finitely many critical points of .
Facts & Assumptions
Given: A closed smooth -manifold , a Morse function and a Riemannian metric on .
The field vanishes exactly at the critical points of , which are finitely many, and at a critical point of index its index is (The Riemannian gradient is the metric dual of the differential, The Riemannian gradient vanishes exactly at the critical points, A Morse gradient zero contributes to the index, Morse functions and excellent Morse functions).
Poincare-Hopf: the index sum of a smooth field with only isolated zeros on a closed manifold equals (Poincare-Hopf for closed manifolds, Euler characteristic of a compact manifold).
Proof
The critical points of a Morse function on a closed manifold are finitely many and each is a nondegenerate zero of ; hence the field has only isolated zeros and [F2] gives .
By [F1] each summand is , so substituting into step 1.1 gives .
Depends on
- Poincare-Hopf for closed manifolds
- A Morse gradient zero contributes $(-1)^\lambda$ to the index
- The Riemannian gradient is the metric dual of the differential
- Every smooth manifold admits a riemannian metric
- Morse functions and excellent Morse functions
- The Riemannian gradient vanishes exactly at the critical points
- Euler characteristic of a compact manifold
- The Axiom of Choice
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- John W. Milnor, Topology from the Differentiable Viewpoint (complete 76-page PDF, including the appendix Classifying 1-manifolds) (standard reference, not scraped)
- Liviu Nicolaescu, An Invitation to Morse Theory (2nd ed.) (standard reference, not scraped)