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Algebraic and topological K-theory, relations with topology, commutative algebra, and operator algebras

23 votes

Magic behind idempotent-complete categories a.k.a. why (sometimes) be Karoubian is sexier th...

I don't know if this is going to answer your question but here's some relevant background. Splitting idempotents has a very special property from a categorical point of view: it is an absolute colimit …
Qiaochu Yuan's user avatar
18 votes
Accepted

Brauer Groups and K-Theory

I'm a bit late to the party, but here's what I suspect the answer should look like. Forgive me for working somewhat to very heuristically throughout. To star things off, here's a silly question: why …
Qiaochu Yuan's user avatar
13 votes
Accepted

What sort of ring-theoretic properties does the representation ring of a compact Lie group p...

Assume that $G$ is connected. Let $T$ be a maximal torus of $G$. Restriction induces a map $R(G) \to R(T)$. Note that $R(T)$ is a Laurent polynomial ring in $r$ variables where $r$ is the rank. Becaus …
Qiaochu Yuan's user avatar
13 votes

What (if anything) unifies stable homotopy theory and Grothendieck's six functors formalism?

I feel like we gave up too quickly on parameterized spectra as a reasonable (if necessarily incomplete) answer to this question; we don't get the two formalisms in the question as special cases of an …
Qiaochu Yuan's user avatar
6 votes

Simplest explicit counterexample for $Vect(BG) \ne Rep(G)$ as monoids

Complex line bundles over $BG$ are classified by $H^1(BG, \mathbb{C}^{\times}) \cong H^2(BG, \mathbb{Z})$. On the other hand, $1$-dimensional complex representations are classified by $\text{Hom}(G, \ …
Qiaochu Yuan's user avatar
6 votes
Accepted

Morita equivalence and isomorphisms in cohomology theories

The conceptual point is that all of these invariants are Morita invariant because they can be defined directly in terms of the category of modules. Explicitly: Starting from the category of modules …
Qiaochu Yuan's user avatar
5 votes
Accepted

Complexification of real k-theory gives index $2$ subgroup of complex k-theory

This is the same as the induced map $\pi_4(KO) \to \pi_4(KU)$. Via Bott periodicity (see also this answer), this is the same as the induced map $\pi_0(KSp) \to \pi_0(KU)$ coming from the map sending a …
Qiaochu Yuan's user avatar
1 vote

Linearized Waldhausen $K$-Theory

Let me make some guesses, although I worry I'll only say some formal things you already know (or that are wrong). One description of the Waldhausen K-theory of a pointed connected space $X$ is that …
Qiaochu Yuan's user avatar