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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 …
some guy on the street's user avatar
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: …
some guy on the street's user avatar
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 …
some guy on the street's user avatar
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 …
some guy on the street's user avatar
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{ …
some guy on the street's user avatar
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 …
some guy on the street's user avatar
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 …
some guy on the street's user avatar
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 …
some guy on the street's user avatar