7.07 Group actions and covering spaces, 2

Below the video you will find accompanying notes and some pre-class questions.

Notes

Reading off the fundamental group

(0.00) Suppose that X is a path-connected space and that x is a basepoint for which π1(X,x)={1}. Suppose that G acts properly discontinuously on X. Then π1(X/G,[x])=G.
(1.15) Let F:Gπ1(X/G,[x]) be the map defined as follows. For each element gG we get a point ρ(g)(x)X. Pick a path γ from x to ρ(g)(x); the projection qγ of this path to the quotient X/G is a loop in X/G based at [x] (because q(ρ(g)(x))=q(x)=[x]). We define F(g)=[qγ].

(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 q1([x], so there exists gG such that ˜δ(1)=ρ(g)(x). This implies that F(g)=[δ].

(5.51) Is F a homomorphism? Given g,hG, 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)].

We have q((ρ(g)γh)γg)=q(ρ(g)γh)q(γg)=q(γh)q(γg)=F(h)F(g),
so F(gh)=F(h)F(g).

(9.44) Therefore F is an antihomomorphism. This is because I defined the concatenation of a followed by b to be ba. 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, xxg, satisfying (xg)h=x(gh). Note that any homomorphism f:GH can be turned into an antihomomorphism ˉf:GH by ˉf(g)=f(g1), so homomorphisms and antihomomorphisms are equally useful.

(11.14) Is F injective? I claim that ker(F) is trivial. If gker(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

(14.00) From this theorem, we can read off the following fundamental groups:
(14.43) Consider the transformations g,h:R2R2 defined by g(x,y)=(x+1,1y)h(x,y)=(x,y+1).
These generate a subgroup G of isometries of R2 and the quotient R2/G is the Klein bottle K. I claim that the action is properly discontinuous and that the group G has the presentation g,h | hg=gh1, which implies that the fundamental group of the Klein bottle is π1(K)=G.

(17.46) To see this, we check gh1(x,y)=g(x,y1)=(x+1,2y)hg(x,y)=h(x+1,1y)=(x+1,2y).

This is the only relation we need in the group because it can be used to put all of the factors of h to the right, so all elements of G can be written as gmhn. This means that any other relation in the group would need to have the form gmhn=1, but we can check that gmhn=1 implies m=n=0. In fact, we can check that gmhn(x,y)=gm(x,y+n)={(x+m,1yn) if m odd(x+m,y+n) if m even.
This equals the identity if and only if m=n=0 and, in fact, d((x,y),gmhn(x,y))1 if (m,n)(0,0),
which implies that the G-action is properly discontinuous.

Pre-class questions

  1. Why is the map F in the proof of the theorem well-defined?
  2. 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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