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Hugo Chapdelaine
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Endomorphisms of degree d of the sphere with infinite fibers

Let $S^n$ be the sphere of dimension $n$. In order to construct a map $f:S^n\rightarrow S^n$ of degree $d\geq 2$ one has the following construction: Let $K$ be the complement of $d$ disjoint n-dimensional open discs in $S^n$. Then collapsing $K$ to a point we find that $S^n/K\simeq \bigvee _{i=1}^d S^n$. Identifying (choosing a homemomorphism) each of the sphere in the disjoint union with $S^n$ (with the approriate orientation), the composition of the two maps $$ S^n\rightarrow \bigvee _{i=1}^d S^n \rightarrow S^n, $$ gives us a map $\phi_d:S^{n}\rightarrow S^n$ of degree $d$.

In general, if $f:S^n\rightarrow S^n$ is a map of degree $d$ and $x\in S^{n}$ is such that the fiber $f^{-1}(x)$ is finite, then one has from excision theorem that $$ \sum_{y\in f^{-1}(x)} deg_f(y)=d. $$

Q1: How would you construct a (continuous) map $f:S^n\rightarrow S^n$ of degree $d$ with a point $x\in S^{n}$ such that the fiber $f^{-1}(x)$ is infinite?

Q2: When the fiber $f^{-1}(x)$ is infinite, is it possible to take some kind natural average sum over the local degrees of the elements of $f^{-1}(x)$ in such a way that the sum converges to $d$ (you may assume that $S^n$ is endowed with a metric if you think it helps) ?

Hugo Chapdelaine
  • 7.6k
  • 2
  • 28
  • 70