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

3 votes

Why is SL(n,Z)[p] modulo the group normally generated by elementary matrices abelian?

I think I can help locating the statement in the Bass-Milnor-Serre paper. First, if I understand your question correctly, the translation between your notation and the one of Bass-Milnor-Serre is $\Ga …
Matthias Wendt's user avatar
4 votes

K-theory of compact Lie groups

Here are some relevant references. First, three papers on different methods to compute the K-theory of compact Lie groups with finite cyclic fundamental group. (The first one also discusses the projec …
Matthias Wendt's user avatar
6 votes
Accepted

Ring structure on K-theory of a quotient of the Fermat quintic

The Atiyah-Hirzebruch spectral sequence does have a multiplicative structure, and I think this can be used to determine the multiplication on K-theory. From the paper of Braun, it follows that the spe …
Matthias Wendt's user avatar
4 votes

Description of higher chow groups

The relation between motivic cohomology and cohomology of the Milnor K-theory sheaf is discussed in Motivic cohomology and cohomology of Milnor K-theory sheaf There is a natural comparison morphism …
Matthias Wendt's user avatar
10 votes

Covering Spaces and Vector Bundles

The category of flat vector bundles is equivalent to the category of local systems, see for instance this MO-question, which in turn are equivalent to representations of the fundamental group, see thi …
Matthias Wendt's user avatar
5 votes
Accepted

Gersten complexes in Quillen's and Milnor's K-theories

Yes, the natural multiplication morphisms induce a morphism of Gersten complexes from Milnor to Quillen K-theory. The basic points are made in the paper M. Rost. "Chow groups with coefficients", Do …
Matthias Wendt's user avatar
10 votes
Accepted

Motivic cohomology and cohomology of Milnor K-theory sheaf

The previous answer contained a major error/misconception, and I apologize for the dealy in correcting it. The answer to the question is "yes" locally in the Zariski topology but "no" globally. Comp …
Matthias Wendt's user avatar
8 votes
Accepted

What is the etale sheafification of the (unramified) Milnor-Witt $K$-theory

We have a short exact sequence of sheaves of abelian groups (see e.g. Morel's book on $\mathbb{A}^1$-algebraic topology): $$ 0\to\mathbf{I}^{n+1}\to\mathbf{K}^{\rm MW}_n\to \mathbf{K}^{\rm M}_n\to 0, …
Matthias Wendt's user avatar
11 votes
Accepted

What is the coefficient ring of algebraic K theory of the discrete $\mathbb{C}$?

I think the answer to this question is not known. All we can say about the K-theory of $\mathbb{C}$ concerns the torsion. The trouble starts with $K_1(\mathbb{C})\cong\mathbb{C}^\times$, which is pre …
Matthias Wendt's user avatar
2 votes
Accepted

$K$-groups and dual graphs of special fibers

I'll draw the connection in the case of where the special fiber $\mathcal{C}_p$ is a triangle of three crossing copies of $\mathbb{P}^1$. Let $Z$ be the closed subscheme of $\mathcal{C}_p$ consisting …
Matthias Wendt's user avatar
19 votes
Accepted

Symplectic K-theory

Concerning the definition of symplectic K-theory: there are various possible definitions, the homotopy groups of the plus-construction of the classifying space of the infinite symplectic group is one …
Matthias Wendt's user avatar
4 votes
Accepted

Is it possible for the Witt group of a scheme to have non-trivial odd torsion?

Examples of smooth real varieties whose Witt group has odd torsion can be found in a paper of Jacobson: J.A. Jacobson. From the global signature to higher signatures. arXiv:1411.0993, https://arxiv …
Matthias Wendt's user avatar
8 votes

Attaching maps for Grassmann manifolds

The degrees of the attaching maps (and hence the integral chain complex) for the real Grassmannians have been determined in L. Casian and Y. Kodama. On the cohomology of real Grassmann manifolds. a …
Matthias Wendt's user avatar
8 votes

$K_0$ of integral group ring of cyclic group $\mathbb{Z}/p\mathbb{Z}$

Let me flesh out my comment, and give some more details. I will denote $C_p$ the cyclic group of order $p$. First, the structure of the group ring $\mathbb{Z}[C_p]$ is a little more complicated. If …
Matthias Wendt's user avatar
4 votes
Accepted

A class in the motivic cohomology group $H^{0,1}(\operatorname{Spec}k;\mathbb{Z}/p)$

The definition of the class is actually given in the cited sentence. For the relevant motivic cohomology group we have $${\rm H}^{0,1}({\rm Spec} k,\mathbb{Z}/p\mathbb{Z})\cong {\rm H}^0_{\rm ét}({\rm …
Matthias Wendt's user avatar

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