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Circle power maps are classified by their exponent
Example
Assume (The Axiom of Countable Choice ()). For let , (equivalently ), with the counterclockwise orientations. Then , and by the Hopf classification for the maps and are homotopic if and only if . Hence the homotopy classes are in bijection with and the class of a power map is determined by its exponent, consistently with the covering-space computation of circle self-maps.
Facts & Assumptions
Given: , the unit circle with its counterclockwise orientation and smooth structure, the real line as its universal cover, and the power maps (Euclidean spheres and closed balls as subspaces of , Smooth manifolds and their smooth charts, is a universal covering).
For every the power map has ; equivalently the continuous map has degree (Degree of the power map on the circle).
For , degree induces a bijection from to , so two maps are homotopic exactly when their degrees agree (Sphere self-maps are homotopic exactly when their degrees agree, Degree of a proper smooth map by compact-support cohomology).
For the quotient covering , the explicit map satisfies and ( is a universal covering). This is an explicit lift of the composite , not a lift of itself.
Verification
By [F1] the degree of is the integer ; the explicit lift of [F3] has period increment , so this increment agrees with the degree already computed by [F1].
Let . If then ; conversely if and are homotopic, [F2] with gives , so the two maps are homotopic exactly when their exponents agree.
Hence the assignment is a bijection from to , inverse to the degree, and a power map is determined up to homotopy by its exponent alone; this matches the covering-space description.
Depends on
- Sphere self-maps are homotopic exactly when their degrees agree
- $\mathbb R\to\mathbb R/\mathbb Z$ is a universal covering
- Degree of a proper smooth map by compact-support cohomology
- Euclidean spheres and closed balls as subspaces of $\mathbb{R}^n$
- Smooth manifolds and their smooth charts
- Degree of the power map on the circle
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
33 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Victor Guillemin and Alan Pollack, Differential Topology (standard reference, not scraped)
- John Milnor, Topology from the Differentiable Viewpoint (standard reference, not scraped)
- Daniel S. Freed, Bordism: Old and New (lecture notes, UT Austin, Fall 2012) (standard reference, not scraped)