7.07 Group actions and covering spaces, 2
Below the video you will find accompanying notes and some pre-class questions.
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Notes
Reading off the fundamental group
(3.33) Is this well-defined? Yes because X is simply-connected (pre-class question: prove this).
(4.04) Is F surjective? Given a loop δ in X/G based at [x], path-lifting gives us a lift ˜δ in X starting at x. The endpoint ˜δ(1) is in q−1([x], so there exists g∈G such that ˜δ(1)=ρ(g)(x). This implies that F(g)=[δ].
(5.51) Is F a homomorphism? Given g,h∈G, pick paths γg (from x to ρ(g)(x) in X) and γh (from x to ρ(h)(x) in X). The path ρ(g)γh connects ρ(g)(x) to ρ(g)ρ(h)(x)=ρ(gh)(x). The concatenation (ρ(g)γh)⋅γg makes sense and provides a path from x to ρ(gh)(x). The projection q((ρ(g)γh)⋅γg) is then a loop in X/G whose lift connects x with ρ(gh)(x), so F(gh)=[q((ρ(g)γh)⋅γg)].
(9.44) Therefore F is an antihomomorphism. This is because I defined the concatenation of a followed by b to be b⋅a. This had the advantage of making monodromy into a homomorphism. To make the current F into a homomorphism, we should use right actions of the group, x↦xg, satisfying (xg)h=x(gh). Note that any homomorphism f:G→H can be turned into an antihomomorphism ˉf:G→H by ˉf(g)=f(g−1), so homomorphisms and antihomomorphisms are equally useful.
(11.14) Is F injective? I claim that ker(F) is trivial. If g∈ker(F) then the loop q(γg) is nullhomotopic (where γg is a path from x to ρ(g)(x) in X). By homotopy lifting, this implies that γg is homotopic to the constant path rel endpoints, which implies that ρ(g)(x)=x. But the action of the group is properly discontinuous, so ρ(g)x=x implies g=1. Therefore the kernel of F is trivial.
Examples
- π1(S1)=Z because S1=R/Z,
- π1(Tn)=Zn because Tn=Rn/Zn,
- π1(RPn)=Z/2 because
RPn=Sn/(Z/2).
(17.46) To see this, we check gh−1(x,y)=g(x,y−1)=(x+1,2−y)hg(x,y)=h(x+1,1−y)=(x+1,2−y).
Pre-class questions
- Why is the map F in the proof of the theorem well-defined?
- In the computation of the fundamental group of the Klein bottle, I
claimed that d((x,y),gmhn(x,y))≥1 if (m,n)≠(0,0).Can you prove this?
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