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Homotopy theory is an important sub-field of algebraic topology. It is mainly concerned with the properties and structures of spaces which are invariant under homotopy. Chief among these are the homotopy groups of spaces, specifically those of spheres. Homotopy theory includes a broad set of ideas and techniques, such as cohomology theories, spectra and stable homotopy theory, model categories, spectral sequences, and classifying spaces.

6 votes
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Does the reduced Mapping cylinder have the same homotopy type of unreduced Mapping cylinder?

I don't trust anything with degenerate basepoints, but I do think you are right about the homotopy type via the parenthetical "(or only using an explicit homotopy)''. However, I was working in compa …
Peter May's user avatar
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3 votes

Does the reduced Mapping cylinder have the same homotopy type of unreduced Mapping cylinder?

I'm lousy at point-set topology, always was, but I don't see that $X$ is closed in $X\wedge I_+$. This is the sort of question a working algebraic topologist does not want to think about. Cofibrant …
Peter May's user avatar
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2 votes
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Path space of a simplicial topological space?

It is sensible to restrict to simplicial {\em based} spaces $X_*$ and then apply the functor $P$ levelwise. This is used, for example, to compare $|\Omega X_*|$ with $\Omega |X_*|$ in The Geometry o …
Peter May's user avatar
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7 votes
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Algebraic $K$-theory of algebras in symmetric spectra: reference

Tom, not precisely sure what you want. I'm guessing you want to think of a commutative symmetric ring spectrum $R$ and then an $R$-algebra $A$. Without the extra layer, just using an $S$-algebra $R$ …
Peter May's user avatar
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2 votes

On the obstruction of a sequence of simplicial spaces that is levelwise a fibration

Working with spaces meaning spaces, I gave a reasonably strong result along these lines in Theorem 12.7 of The Geometry of Iterated Loop Spaces http://www.math.uchicago.edu/~may/BOOKS/geom_iter.pdf. …
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4 votes

Homology of homotopy fixed point spectra

Maybe to expand on the relationship between naive and genuine G-spectra, Tom's answer is stated in terms of naive G-spectra, but the actual mathematics, the Segal conjecture, is all about genuine G-sp …
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7 votes
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Does a pointed homotopy equivalence between pointed $G$-spaces which is $G$-equivariant indu...

In the pointed Borel construction, you clearly mean $\wedge$ and not $\times$. Thus $$EG_+\wedge_G X = EG\times_G X/EG\times_G\ast.$$ Out of laziness, I'll assume that your $X$ and $Y$ are of the $G …
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7 votes

Coherent MU_*-Modules

The question is also addressed in Lecture 5 of J.F. Adams ``Lectures on generalized cohomology'' in Springer Lecture Notes in Mathematics Vol 99(1969). Again ancient, but none the worse for that. Th …
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13 votes

Is a map a homotopy equivalence if its suspension is so?

Since adequate answers to the question asked have been given, I will address the relevant underlying theorems. There are two relevant theorems, both called ``Whitehead's theorem''. One says that a w …
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19 votes
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Computing homotopies

Sometimes easy geometric pictures have awkward seeming algebraic descriptions. On pages 6 and 7 of Concise, I gave examples where I both gave a geometric picture and explicit formulas to make the idea …
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2 votes

The homotopy cofiber of the smash product of two maps of spectra

An axiomatization of exactly how smash products of cofiber sequences should behave is given in the context of triangulated categories in my paper The additivity of traces in triangulated categories …
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9 votes
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$\mathcal{I}$-functors and infinite loop spaces

You are looking at the telescope of maps $BG_n\longrightarrow BG_{n+1}$ where the coproduct of the $G_n$ (thought of as categories) has a structure of permutative category. The group completion prope …
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0 votes

A homotopyish Landweber exact functor theorem

Akhil, short though that paper is, I claim that the proof there is as complete as it needs to be.
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6 votes
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units in non-commutative ring spectra

The notation $\Omega^{\infty}$ requires care. Definitions and comparisons for symmetric, orthogonal, and EKMM spectra are given in Lind's thesis. See https://arxiv.org/abs/0908.1092. For symmetric …
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22 votes

Why study the p-completions of a space?

I'll give an answer from the point of view of an algebraic topologist, somebody who cares about examples and computations. This all goes way back and has nothing to do with modern generalities. Fir …
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