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Complex structure on an associated bundle by realification

Physical scope

Pure mathematical well-definedness result; it has no physical assumptions or empirical claim.

Statement

Let H be a finite-dimensional real Lie group, let π:P→M be a smooth right principal H-bundle, let W be a finite-dimensional complex vector space, and let ρ:H→GLC(W) be a smooth complex representation. Regard W as the underlying real vector space WR and ρ as the real representation ρR. The associated quotient P×H,ρRWR has the smooth real vector-bundle structure supplied by the associated-vector-bundle theorem. The map J[p,w]=[p,iw] is well-defined and smooth and satisfies J2=−id. It therefore gives this real bundle a complex vector-bundle structure of rank dim⁡CW, with transition maps ρ(gji) in any local principal trivializations si=sjgji. This assertion specifies the complex structure induced by J and makes no uniqueness claim about other possible complex structures on the same real bundle.

Facts & Assumptions

Given: A smooth right principal H-bundle P→M, a finite-dimensional complex vector space W, and a smooth complex representation ρ.

[F1]

Principal g bundle and associated fiber bundle: the associated quotient uses the relation (ph,w)∼(p,ρ(h)w).

[F2]

Associated vector bundles are well-defined: for a smooth right principal bundle and a smooth finite-dimensional real representation, this quotient has a smooth real vector-bundle structure whose transition maps are the representation matrices of the principal transition functions.

Proof

1.1F2givenalgebra

The underlying real space WR is finite-dimensional. In a complex basis, the inclusion GLC(W)↪GLR(WR) sends a matrix A to the real block matrix (Re⁡A−Im⁡AIm⁡ARe⁡A); its entries depend linearly on the real and imaginary parts of those of A, so the inclusion is smooth. Therefore ρR is a smooth real representation. Applying [F2] supplies the smooth real vector-bundle structure and the local charts induced by sections si:Ui→P, with transition maps ρR(gji).

2.1F1F2givenstep 1.1algebra∎

If (ph,w)∼(p,ρ(h)w), then J[ph,w]=[ph,iw]=[p,ρ(h)iw]=[p,iρ(h)w]=J[p,ρ(h)w], because ρ(h) is complex-linear. Thus J is well-defined on the quotient. In the local charts from step 1.1 it acts by (x,w)↦(x,iw), so it is smooth and J2=−id. Each fiber consequently has the complex scalar action (a+ib)[p,w]=[p,aw+ibw]. The chart transitions ρ(gji) commute with multiplication by i, so they are complex-linear; the resulting complex bundle has rank dim⁡CW.

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