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ALGTOP · 1

Homotopy

4 concepts · 15 questions

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ALGTOPStep●○○○○new1 Homotopy
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The problem

Let X=C∖{1,−1}X = \mathbb{C}\setminus\{1, -1\} with basepoint 00. For a loop γ\gamma in XX let w±(γ)w_{\pm}(\gamma) be the degree of (γ−(±1))/∣γ−(±1)∣(\gamma - (\pm1))/|\gamma - (\pm1)|, the winding number of γ\gamma about ±1\pm1. (a) Show that W:π1(X,0)→Z2W : \pi_1(X, 0)\to\mathbb{Z}^2, [γ]↦(w+(γ),w−(γ))[\gamma]\mapsto(w_+(\gamma), w_-(\gamma)), is a well-defined homomorphism. (b) Show that WW is surjective. (c) Deduce that XX is not contractible and not homotopy equivalent to S1S^1.

Why is w+w_+ a homomorphism π1(X,0)→Z\pi_1(X, 0)\to\mathbb{Z}?