Induced map on associated bundles

How a principal bundle morphism induces a map between associated bundles.
Induced map on associated bundles

Let PMP\to M and PMP'\to M' be . A principal bundle morphism consists of a smooth map f:MMf:M\to M' and a smooth map Φ:PP\Phi:P\to P' such that

πΦ=fπ,Φ(pg)=Φ(p)g  for all gG. \pi'\circ \Phi = f\circ \pi,\qquad \Phi(pg)=\Phi(p)g\ \text{ for all }g\in G.

Let FF and FF' be left GG-spaces, and let ψ:FF\psi:F\to F' be GG-equivariant.

Construction. Define

Φ×Gψ:P×GFP×GF,[p,u][Φ(p),ψ(u)]. \Phi\times_G \psi : P\times_G F \to P'\times_G F',\qquad [p,u]\mapsto [\Phi(p),\psi(u)].

This is well-defined (independent of the representative (p,u)(p,u)) and is a smooth bundle map covering ff:

πF(Φ×Gψ)=fπF. \pi_{F'}\circ (\Phi\times_G\psi) = f\circ \pi_F.

When F=FF=F' and ψ=id\psi=\mathrm{id}, one gets the induced map on a fixed associated bundle functorially from Φ\Phi.

Examples

  1. (Pullback trivialization maps) If P=PP'=P and f=idMf=\mathrm{id}_M, then a gauge transformation Φ\Phi induces a bundle automorphism of any P×GFP\times_G F by [p,u][Φ(p),u][p,u]\mapsto [\Phi(p),u].
  2. (Vector bundle case) For F=VF=V and F=VF'=V' linear GG-spaces, a GG-equivariant linear map ψ:VV\psi:V\to V' yields a vector bundle morphism P×GVP×GVP\times_G V\to P'\times_G V' covering ff.
  3. (Adjoint functoriality) Taking F=F=GF=F'=G with conjugation and ψ=id\psi=\mathrm{id} gives a morphism of induced from any principal bundle morphism.