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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.

18 votes

cohomology of BG, G compact Lie group

Here's the argument I know that avoids spectral sequences, based on the little-known space $G/N(T)$. In between $T$ and $G$ is $N(T)$. Note that $EG$ "is an" $ET$ and $EN(T)$, since it's contractib …
Allen Knutson's user avatar
1 vote
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Elementary question: Intuition for equivariant cohomology

I'm guessing that, unstated, $M,G$ are finite-dimensional and $G$ is connected Lie. Then $H^*(M/G)$ vanishes for $* \gg 0$, but $H^*_G$ is positively graded, so $H^*(M/G)$ must be a torsion module. …
Allen Knutson's user avatar