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Homotopy theory, homological algebra, algebraic treatments of manifolds.
13
votes
0
answers
781
views
What's so difficult about $\pi_{15}(SO)$?
Regarding the table of $SO(n)$s-of-origin in Davis+Mahowald (if you can get MathSciNet), is there a good reason that it should take longer for $\pi_{15}(SO)$ to be representable than $\pi_{19}(SO)$, w …
12
votes
Accepted
Geometric meaning of torsion in homotopy groups
Well, the silly answer is that $f:\mathbb{S}^k\to X$ represents a torsion element of order $p$ if $p \cdot f:\mathbb{S}^k\to X$ extends along $\mathbb{S}^k \hookrightarrow D^{k+1}$ to a map $\varphi: …
12
votes
4
answers
2k
views
The most general context of Mather's Cube Theorems
Quite simply, I'd like to know what is the broadest or most natural context in which either (or both) of Mather's cube theorems hold. If you like, this may mean any of
What properties of $Top$ or $T …
5
votes
Why should I prefer bundles to (surjective) submersions?
This is probably making a hash of the earlier answers, but bundles are special fibrations; specifically, they are fibrations with (not canonically) isomorphic fibers. And we all like fibrations, righ …
5
votes
Accepted
What is the precise relationship between "prodsimplicial sets" and rooted trees?
There are a short list of operations described as generating the desired polyhedra:
$ X : \mathrm{Prism} \vdash C X : \mathrm{Prism} $
$ l : \mathrm{list}\ \mathrm{Prism} \vdash \Pi l : \mathrm{ …
4
votes
1
answer
241
views
Topological Localization of (the simply-connected cover of) SO or Spin
This is essentially a (cw) reference request, because it seems like the sort of thing that should have been looked at already.
Setting aside, for now, how to think what the localization of a general …
3
votes
Associativity with infinite nesting
First, it's important that the infinite connect sum $A\# B\#A\#\cdots$ is not the limit of the finite connect sums $A,A\#B, A\#B\# A,\dots$; in fact, I'm sure the binary connect sum is as wrong a nota …
2
votes
A conceptual proof that local fibrations over paracompact spaces are global fibrations?
The following runs out of steam towards the end; I may also be making important mistakes, so be on your guard --- but that's half the fun! Anyways, it was too long for a comment.
Choose a locally-fi …
0
votes
Examples where it's useful to know that a mathematical object belongs to some family of objects
I have two related sorts of example to suggest, probably exhibiting my categorical bias vs. the analytic/geometric-topology weight of the preceding examples.
Galois-theoretic
Let $P\in K[x]$ be an i …
0
votes
A canonical and categorical construction for geometric realization
I'd just like to point out that there is a monad on $Top$, (which in the homotopy category looks rather dull,) assigning to each space $X$ its cone $CX$, the mapping cylinder of $X\to * $. The unit m …