# Questions tagged [algebraic-k-theory]

Algebraic K-theory is a tool from homological algebra that defines a sequence of functors from rings to abelian groups. It has many applications in algebraic geometry. See also (topological-k-theory).

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### The definition of (special) $\lambda$-rings by using the ring $W(K)$ of big Witt vectors.

I am reading C. Weibel's 'The K-book', the definition of (special) $\lambda$-rings (in Page 102, Definition 4.3.1) is A pre-$\lambda$-ring $(K,+,\cdot)$ is called a $\lambda$-ring, if the following ...
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1 vote
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### The multiplication '$*$' of the ring $W(K)=1+tK[[t]]$ in C.Weibel's 'The K-book'

I am reading C. Weibel's 'The K-book', and in page 101, example 4.3, there is a construction of the ring $(W(K)=1+tK[[t]],\cdot,*)$. I could not understand how can we construct the multiplication '$*$'...
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### Minimal polynomial of $\mathcal{O}(1)\otimes -$-operator

This is somewhat of a follow-up to this question. Take $X= \mathbb{P}^1, G = \operatorname{GL}(2)$. Compute the minimal polynomial of the operator $\mathcal{O}(1)\otimes -$ on $K_G(X)$. How does one ...
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### Pushforward of a line bundle on $\mathbb{P}^1$ to a point

In his notes on K-theory, A. Okounkov states the following exercise: The group $\operatorname{GL}(2)$ acts naturally on $\mathbb{P}^1$ and on line bundles $\mathcal{O}(k)$ over it. Push forward these ...
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### Showing $K_0(F)=\mathbb{Z}$

In Morel's "$\mathbb{A}^1$-algebraic topology over a field", Milnor-Witt K-theory is defined to be the graded ring $K_*^{MW}(F)$ generated by the symbols, $[u]$ of degree $1$ and $\eta$ of ...
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### What does $p+q \leq 1$ means in the context of $C^*$-algebra?

Let $A$ be a $C^*$-algebra, and let $p,q \in A$. The book I'm following(An Introduction to K-theory for $C^*$-algebra) has this notation $p+q \leq 1$. What does this mean? I looked through the book, ...
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1 vote
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### Is a topological space that is homotopy equivalent to a CW-complex locally path-connected?

While describing Quillen’s $+$-construction in his book Algebraic K-Theory, V. Srinivas assumes that his topological spaces are equivalent to CW-complexes and are path-connected. As universal ...
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### Direct Proof that Artinian Rings have Stable Range 1

Is there a direct proof that any (right) Artinian ring has stable range 1? More precisely, let $R$ be a right Artinian ring and $a,b\in R$ be such that $aR+bR=R$. Can we prove that $(a-bt)R=R$ for ...
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### Mayer-Vietoris Exact Sequence for Whitehead groups and projective module groups

I came across the claim that there is an exact sequence $K_1\Lambda\to K_1\Lambda_2\oplus K_1\Lambda_2\to K_1\Lambda'\to K_0\Lambda\to K_0\Lambda_1\oplus K_0\Lambda_2\to K_0\Lambda'$ while reading ...
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### Lambda operations in K-theory

In Weibel's K-Book, before defining the lambda-operations on higher (Quillen) K-theory, he states that "Although many constructions of $\lambda$-operations have been proposed in more exotic ...
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### Etale $K$-theory of finite fields.

Do etale $K$-theory of finite fields coincide with algebraic $K$-theory? what is the easiest way to see this (I think etale and algebraic $K$-theory coincide above a certain value like cohomological ...
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### Chow group of a specific scheme related to the localization of the affine line at two points.

Given a scheme $X$, let $S=\varprojlim\limits_{U\subset X\times \mathbb{A}^1}U$ where $U$ is a Zariski open that contains both of $X\times\{1\}$ and $X\times\{0\}$. What does the Chow group $CH^i(S)$ ...
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### Counterexamples to the Grothendieck group of a commutative monoid respecting tensor products

$\newcommand{\Q}{\mathbb{Q}}\newcommand{\N}{\mathbb{N}}\newcommand{\Z}{\mathbb{Z}}$Recall the definitions of the tensor product $\otimes_{\mathbb{N}}$ of commutative monoids (see also this note by ...
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1 vote
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### Show the spectral sequence of a fibre sequence is a spectral sequence of algebras

I'm reading Notes on the K-theory of Finite Fields by Steve Mitchell. Here is a lemma from this paper. We want to show that the maps in the spectral sequence of the fibre sequence is $\mathbb{Z}/l$ -...
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### Where and by whom is the stable rank first defined?

To the best of my knowledge, Bass defined the notion of stable range. Then somewhere it was shown that the condition on unimodular sequences defining the stable range holds for $N\geq n$ if it holds ...
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### Intuition behind 4-fold periodicity of $L$-theory

The quadratic and the symmetric L-groups are 4-fold periodic. What is the simple argument to obtain the intuition behind the 4-fold periodicity of $L$-theory? (For example, why not have the Bott ...
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1 vote
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### A reference question on the Dedekind zeta function and other L-functions.

On page 501 in the following ICM1983 conference proceeding https://www.mathunion.org/fileadmin/ICM/Proceedings/ICM1983.1/ICM1983.1.ocr.pdf you will find a conjecture on the L-function $L(X,s)$ of an ...
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### Note or textbook about algebraic k-theory

I'm reading the Weibel's K-book. It's difficult for me. I have read some $K_0$ with the hlep of Bass' book. But Bass' book is so old. So, are there any more detailed books or notes to recommend. I ...
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### Does isomorphism at the level of residue fields imply isomorphism of etale $K$-theory?

I am wondering whether something like the following argument is correct or not? If a morphism of regular schemes $f:X\rightarrow Y$ induces an isomorphism on residue fields and also induces a ...
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### Bott inverted algebraic $K$-theory vs algebraic $K$-theory.

Bott inverted mod $l$ algebraic $K$-theory coincides with mod $l$ etale $K$-theory under mild conditions. The Bott element is an element in $K_2$ that is inverted with respect to the graded product ...
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1 vote
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### Restriction of $G$-equivariant sheaves to fixed locus

Let $G$ be a group acting on a Noetherian scheme $X$ over $\mathbb{C}$ and $\mathcal{E}$ be a $G$-equivariant coherent sheaf on $X$ where the support of $\mathcal{E}$ contains the fixed locus $X^G$. I ...
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### When existence of transfer map of algebraic $K$-theory implies rational injection.

Given a map of smooth projective varieties $f:X\rightarrow Y$ over fields, there is a projection formula in algebraic $K$-theory given by $f_*(\alpha.f^*(\beta))=\beta.f_*(\alpha)$. I was wondering ...
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### $K$-theory of formal power series.

I was wondering whether there is a calculation of algebraic $K$-groups of the formal power series $\mathbb{F}_p[[x]]$?
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### $K$-theory of union of smooth curves.

Is there a method that one can calculate algebraic $K$ theory of a number of smooth curves with some singularity points corresponding to the intersection points of two different curves, given we know ...
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1 vote
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### $\mathcal{K}$-homology sheaf on affine space.

The $n$-th $\mathcal{K}$-homology sheaf on $X$ is defined as the sheafification of the presheaf $U\mapsto K_n(U)$. Here $K_n(U)$ is the $n$-th algebraic $K$-group. I was just wondering whether it is ...
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### $G_1$ of a nodal projective curve.

Let $C$ be the projective nodal curve over the field $k$ i.e. the projective line with two points identified. I'm having trouble calculating the $G$-theory of $C$, especially $G_1$. Writing the ...
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### K Theory: Book Recommendations

Good people! So I've been hoping to get into K Theory for a while now, and the book that I have been trying to use (and failing) has been Charles Weibel's book by that very title. The book itself isn'...
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### Elementary computations for hermitian and non-hermitian algebraic K-theory

As the title states, I am looking for some examples of non trivial albeit elementary calculations of algebraic hermitian and non-hermitian $K$-theory groups. Concretely, I would be interested in ...
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### Is the nerve of a symmetric monoidal category a K-theory space?

It's an amazing fact that if the $C$ is a symmetric monoidal category so that its components form a group, then $NC$ is an infinite loop space. Now if we have a Waldhausen category $D$, a category ...
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### $K$-group of category of bounded chain complexes of free modules with finite length homologies

For a Noetherian local ring $(R, \mathfrak m)$, let $K_0^{\mathfrak m}(R)$ denote the abelian Group generated by isomorphism classes of bounded chain complexes of finitely generated free modules with ...
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