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The classifying space BG of a group G classifies principal G-bundles, in that homotopy classes of maps [X, BG] are naturally identified with isomorphism classes of principal G-bundles P ⭢ X.

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When does $BG \to BA$ loop to a homomorphism?

If $G$ is a compact connected topological group and $A$ is a locally compact abelian topological group, then for any map $f:BG\to BA$ the looped map $\Omega f:\Omega BG\to \Omega BA$ is homotopically …
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8 votes
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rationalization of classifying spaces

In the paper Arkowitz, Martin Categories equivalent to the category of rational H-spaces, Manuscripta Math. 64 (1989), no. 4, 419–429 it is shown that the rational homotopy equivalence $G_\mathbb{Q} …
Mark Grant's user avatar
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11 votes
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Integral versus real (universal) characteristic classes

If $X$ is a space of finite type (meaning that the homology groups $H_i(X)$ are all finitely generated, a condition which applies in particular to $X=BG$ for $G$ a compact Lie group) then for each $n$ …
Mark Grant's user avatar
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2 votes

Equivariant Cohomology for actions with finite stabilizers

In fact "$\mathbb{Q}$-acyclic" means "having the same rational homology groups as a point". In particular, the classifying space of any finite group is $\mathbb{Q}$-acyclic, as can be seen by a simple …
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8 votes
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Dualizable classifying spaces

Any finitely presented group of type FL admits a finite classifying space. (A group $G$ is of type FL if $\mathbb{Z}$ admits a finite length resolution by finitely generated, free $\mathbb{Z}G$-module …
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17 votes

cohomology of BG, G compact Lie group

If you know a bit of Algebraic Topology (in particular the dreaded spectral sequences), the following is a nice way to see this. The well known Hopf Theorem states that for $G$ a compact connected Li …
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