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Smooth principal bundle from two-chart gluing data

Statement

Let X be a Hausdorff, second-countable smooth manifold covered by open sets U and V. Let G be a finite-dimensional, Hausdorff, second-countable real Lie group, and let τ:U∩V→G be smooth. On the disjoint union Q=(U×G)U⊔(V×G)V, impose the equivalence relation generated, for x∈U∩V and h∈G, by

(x,h)V∼(x,τ(x)h)U.

Let P:=Q/∼ with the quotient topology, write [x,h]U and [x,h]V for its classes, and define π:P→X by π([x,h]U)=π([x,h]V)=x. Define the right action by [x,h]i⋅k=[x,hk]i for k∈G and i∈{U,V}. Then P is a smooth right principal G-bundle over X. Its local sections sU(x)=[x,e]U and sV(x)=[x,e]V satisfy sV(x)=sU(x)τ(x) on U∩V.

Facts & Assumptions

Given: The Hausdorff, second-countable smooth manifold X, its open cover U,V, the finite-dimensional Hausdorff, second-countable real Lie group G, and the smooth transition map τ.

[A1]

The spaces U and V are open subsets of X, cover X, and inherit its Hausdorff, second-countable smooth-manifold structure; τ is smooth on their overlap.

[F1]

A smooth right principal G-bundle has an underlying topological right principal bundle and a smooth projection and right action, with equivariant diffeomorphisms from the inverse images of base charts to product charts. Smooth principal G-bundle

[F2]

Multiplication and inversion in a finite-dimensional real Lie group are smooth. Lie group

Proof

1.1givenA1F2algebra

The generated equivalence classes contain one representative from each chart copy exactly when the base point is in U∩V: the transition (x,h)V↦(x,τ(x)h)U is bijective there, with inverse (x,k)U↦(x,τ(x)−1k)V by [F2]. Thus the canonical maps jU:U×G→P and jV:V×G→P are injective. For every open W⊆U×G, the preimage of jU(W) in Q is the open subset WU together with the inverse transition image of W∣(U∩V)×G in the V copy; hence jU(W) is open in the quotient. The same argument applies to jV, so these maps are open embeddings and their images give product charts covering P. On their overlap jU−1jV(x,h)=(x,τ(x)h), a diffeomorphism with inverse (x,k)↦(x,τ(x)−1k) by [F2]. Therefore these product charts define a smooth atlas on P.

2.1givenA1step 1.1

The induced projection is continuous because the two product projections agree on the equivalence relation. If two points of P project to distinct points of X, disjoint base neighborhoods separate them since X is Hausdorff. If they project to the same x, choose U or V containing x; both points lie in that product chart, where distinct group coordinates have disjoint neighborhoods because the chart is Hausdorff. Hence P is Hausdorff. Each product chart is second-countable, and the union of countable bases from the two charts is a countable base for P. The coordinate changes are smooth by step 1.1, so the resulting smooth manifold structure has the required separation and countability properties.

3.1givenA1F1F2step 1.1step 2.1algebra∎

The formula [x,h]i⋅k=[x,hk]i is well-defined because (x,τ(x)h)U⋅k=(x,τ(x)hk)U∼(x,hk)V; associativity and the identity law follow from the group law. In each chart it is the smooth product action by [F2], and it is free and transitive on each fibre because right multiplication on the G coordinate has those properties. By steps 1.1 and 2.1, the charts are smooth product charts and P is Hausdorff and second-countable. The map π is smooth and locally the product projection, so [F1] applies and P→X is a smooth right principal G-bundle. The sections sU,sV are smooth in these charts, and [x,e]V=[x,τ(x)]U gives sV=sUτ.

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