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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.

48 votes
6 answers
4k views

Why the "W" in CGWH (compactly generated weakly Hausdorff spaces)?

In his 1967 paper A convenient category of topological spaces, Norman Steenrod introduced the category CGH of compactly generated Hausdorff spaces as a good replacement of the category Top topological …
André Henriques's user avatar
36 votes
0 answers
1k views

Functor that maps to both $KO^n$ and $KO^{-n}$

(my question is also meaningful for complex K-theory, but since Kn(X) is always isomorphic to K-n(X), it's less interesting) I start by recalling the analytic definition of KO-theory: The following …
André Henriques's user avatar
36 votes
3 answers
2k views

Defining $SU(n)$ in HoTT

From a recent answer by Mike Shulman, I read: "HoTT is (among other things) a foundational theory, on roughly the same ontological level as ZFC, whose basic objects can be regarded as $\infty$-gro …
André Henriques's user avatar
25 votes
2 answers
960 views

Is super-vector spaces a "universal central extension" of vector spaces?

Is there some sense in which the category $sVect$ of super-vector spaces is the "maximal non-trivial extension" of $Vect$ as a symmetric monoidal category? Is the $\mathbb Z/2$ that shows up in the d …
André Henriques's user avatar
19 votes
3 answers
1k views

Are Chern classes well defined up to contractible choice?

The Chern classes are, by definition, cohomology classes. And cocycle representatives of the Chern classes are not unique. But it might be the case that cocycle representatives of the Chern classes ar …
André Henriques's user avatar