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Homotopy theory, homological algebra, algebraic treatments of manifolds.

13 votes

Can both G and BG be finite CW complexes?

As stated in previous answers, the answer to the question is no. Here is another viewpoint, which gives additional information about the relationship between $G$ and $BG$: A pair $(G,BG)$ where $G$ …
Jesper Grodal's user avatar
4 votes
Accepted

Homology of homotopy fixed point spaces

Hej Craig, Re (2) as Tilman says in his comment, there is an unstable homotopy fixed point spectral sequence, a special case of the spectral sequence of a homotopy limit as described by Bousfield and …
Jesper Grodal's user avatar
11 votes
Accepted

Homology of manifold with action of group

No, this is in general quite far from being true (and also somewhat ill-posed, since there is not a natural candidate for what "=" means since there is no map). For example, take $M = S^1$ and $N=S^2 …
Jesper Grodal's user avatar
11 votes

When is the quotient by an $n$-fold loop space an $m$-fold loop space?

Even for $n=1$ the cofiber will almost never be a loop space. There is no reason for it to be a loop space, and plenty of reasons for it not to be a loop space. E.g., it would need to be a simple s …
Jesper Grodal's user avatar
12 votes

Dimension of a homotopy type

If you are willing to make "niceness" assumptions, assuming that the n-type is, say, nilpotent with finite fundamental group, then the dimension, in any reasonable sense, will always be infinite, sinc …
Jesper Grodal's user avatar
11 votes

Relation between groups and classifying spaces

The space $BG/G$, with the model you describe, has been studied, but is not completely understood. It's $\pi_1$ identifies with the abelianization, but there may be higher homotopy. It identifies wi …
Jesper Grodal's user avatar
6 votes

When are all centralizers in a Lie group connected?

Centralizers of various types of subgroups, and their relationship to invariants like the cohomology of the classifying space $BG$ has a long history of study, at least going back to the papers from t …
Jesper Grodal's user avatar
6 votes

Why localize spaces with respect to homology?

I'm not sure exactly what you are after, but here is an elementary discussion, surely well known to you. Rephrasing what Mark Hovey said, the first question you should ask yourself is probably why loc …
Jesper Grodal's user avatar
8 votes
Accepted

Computing the homology groups of spaces in a fibration

Repeating Mark Grant's comment, the spectral sequence when all spaces are $K(\pi,1)$s goes under the name Lyndon-Hochschild-Serre spectral sequence. Good references for this spectral sequence are: …
Jesper Grodal's user avatar
12 votes

Weight lattice and the fundamental group

A good reference for 1) is Bourbaki: Lie groups and Lie algebras Chapter 9. See in particular Section 4.6. In particular it follows that 2) $\pi_1(G/T) = 0$ and that 4) $\pi_1(G)$ is finite if and o …
Jesper Grodal's user avatar
14 votes
Accepted

Homotopy properties of Lie groups

The problem you mention has a long history. The best homotopy characterization is probably using the notion of finite loop spaces: A finite loop space is a space $BG$ such that $\Omega BG$ is homotopy …
Jesper Grodal's user avatar
14 votes
Accepted

$A_{\infty}$-structure on closed manifold

Yes: As has been pointed out, admitting the structure of a connected $A_\infty$-space is the same thing as being homotopy equivalent to a connected loop space. The Hilton-Roitberg criminal, mentione …
Jesper Grodal's user avatar