We generalize a result, on the pro-representability of Milnor $K$-cohomology groups at the identity, that's due to Bloch. In particular, we prove, for $X$ a smooth, proper, and geometrically connected variety defined over an algebraic field extension $k/mathbb{Q}$, that the functor [mathscr{T}_{X}^{i,3}(A)=kerleft(H^i(X_A,mathcal{K}_{3,X_A}^M)rightarrow H^i(X,mathcal{K}_{3,X}^M)right),] defined on Artin local $k$-algebras $(A,mathfrak{m}_A)$ with $A/mathfrak{m}_Acong k$, is pro-representable provided that certain Hodge numbers of $X$ vanish.
{"title":"Prorepresentability of KM-cohomology in\u0000weight 3 generalizing a result of Bloch","authors":"Eoin Mackall","doi":"10.2140/akt.2023.8.127","DOIUrl":"https://doi.org/10.2140/akt.2023.8.127","url":null,"abstract":"We generalize a result, on the pro-representability of Milnor $K$-cohomology groups at the identity, that's due to Bloch. In particular, we prove, for $X$ a smooth, proper, and geometrically connected variety defined over an algebraic field extension $k/mathbb{Q}$, that the functor [mathscr{T}_{X}^{i,3}(A)=kerleft(H^i(X_A,mathcal{K}_{3,X_A}^M)rightarrow H^i(X,mathcal{K}_{3,X}^M)right),] defined on Artin local $k$-algebras $(A,mathfrak{m}_A)$ with $A/mathfrak{m}_Acong k$, is pro-representable provided that certain Hodge numbers of $X$ vanish.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2023-01-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47579627","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Witt ring of symmetric bilinear forms over a field has divided power operations. On the other hand, it follows from Garibaldi-Merkurjev-Serre's work on cohomological invariants that all operations on the Witt ring are essentially linear combinations of exterior powers. We find the explicit formula for the divided powers as a linear combination of exterior powers. The coefficients involve the ``tangent numbers'', related to Bernoulli numbers. The divided powers on the Witt ring give another construction of the divided powers on Milnor K-theory modulo 2.
{"title":"Divided powers in the Witt ring of symmetric bilinear forms","authors":"B. Totaro","doi":"10.2140/akt.2023.8.275","DOIUrl":"https://doi.org/10.2140/akt.2023.8.275","url":null,"abstract":"The Witt ring of symmetric bilinear forms over a field has divided power operations. On the other hand, it follows from Garibaldi-Merkurjev-Serre's work on cohomological invariants that all operations on the Witt ring are essentially linear combinations of exterior powers. We find the explicit formula for the divided powers as a linear combination of exterior powers. The coefficients involve the ``tangent numbers'', related to Bernoulli numbers. The divided powers on the Witt ring give another construction of the divided powers on Milnor K-theory modulo 2.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-09-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42513899","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"On classification of nonunital amenable simple\u0000C∗-algebras, III : The range and the reduction","authors":"G. Gong, Huaxin Lin","doi":"10.2140/akt.2022.7.279","DOIUrl":"https://doi.org/10.2140/akt.2022.7.279","url":null,"abstract":"","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41490142","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Demba Barry, Alexandre Masquelein, Anne Qu'eguiner-Mathieu
. Using the Rost invariant for non split simply connected groups, we define a relative degree 3 cohomological invariant for pairs of orthogonal or unitary involutions having isomorphic Clifford or discriminant algebras. The main purpose of this paper is to study general properties of this invariant in the unitary case, that is for torsors under groups of outer type A . If the underlying algebra is split, it can be reinterpreted in terms of the Arason invariant of quadratic forms, using the trace form of a hermitian form. When the algebra with unitary involution has a symplectic or orthogonal descent, or a symplectic or orthogonal quadratic extension, we provide comparison theorems between the corresponding invariants of unitary and orthogonal or symplectic types. We also prove the relative invariant is classifying in degree 4, at least up to conjugation by the non-trivial automorphism of the underlying quadratic extension. In general, choosing a particular base point, the relative invariant also produces absolute Arason invariants, under some additional condition on the underlying algebra. Notably, if the algebra has even co-index, so that it admits a hyperbolic involution, which is unique up to isomorphism, we get a so-called hyperbolic Arason invariant. Assuming in addition the algebra has degree 8, we may also define a decomposable Arason invariant. It generally does not coincide with the hyperbolic Arason invariant, as the hyperbolic involution need not be totally decomposable.
{"title":"Degree 3 relative invariant for unitary\u0000involutions","authors":"Demba Barry, Alexandre Masquelein, Anne Qu'eguiner-Mathieu","doi":"10.2140/akt.2022.7.549","DOIUrl":"https://doi.org/10.2140/akt.2022.7.549","url":null,"abstract":". Using the Rost invariant for non split simply connected groups, we define a relative degree 3 cohomological invariant for pairs of orthogonal or unitary involutions having isomorphic Clifford or discriminant algebras. The main purpose of this paper is to study general properties of this invariant in the unitary case, that is for torsors under groups of outer type A . If the underlying algebra is split, it can be reinterpreted in terms of the Arason invariant of quadratic forms, using the trace form of a hermitian form. When the algebra with unitary involution has a symplectic or orthogonal descent, or a symplectic or orthogonal quadratic extension, we provide comparison theorems between the corresponding invariants of unitary and orthogonal or symplectic types. We also prove the relative invariant is classifying in degree 4, at least up to conjugation by the non-trivial automorphism of the underlying quadratic extension. In general, choosing a particular base point, the relative invariant also produces absolute Arason invariants, under some additional condition on the underlying algebra. Notably, if the algebra has even co-index, so that it admits a hyperbolic involution, which is unique up to isomorphism, we get a so-called hyperbolic Arason invariant. Assuming in addition the algebra has degree 8, we may also define a decomposable Arason invariant. It generally does not coincide with the hyperbolic Arason invariant, as the hyperbolic involution need not be totally decomposable.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45103964","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this paper we give a purely categorical construction of d-fold matrix factorizations of a natural transformation, for any even integer d. This recovers the classical definition of those for regular elements in commutative rings due to Eisenbud. We explore some natural functors between associated triangulated categories, and show that when d=2 these are full and faithful, and in some cases equivalences.
{"title":"Categorical matrix factorizations","authors":"P. A. Bergh, David A. Jorgensen","doi":"10.2140/akt.2023.8.355","DOIUrl":"https://doi.org/10.2140/akt.2023.8.355","url":null,"abstract":"In this paper we give a purely categorical construction of d-fold matrix factorizations of a natural transformation, for any even integer d. This recovers the classical definition of those for regular elements in commutative rings due to Eisenbud. We explore some natural functors between associated triangulated categories, and show that when d=2 these are full and faithful, and in some cases equivalences.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46732836","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
. The present paper is a continuation of earlier work by Gunnar Carlsson and the first author on a motivic variant of the classical Becker-Gottlieb transfer and an additivity theorem for such a transfer by the present authors. Here, we establish a motivic variant of the classical Segal-Becker theorem relating the classifying space of a 1-dimensional torus with the spectrum defining algebraic K-theory.
{"title":"The motivic Segal–Becker theorem for algebraic\u0000K-theory","authors":"R. Joshua, Pablo Peláez","doi":"10.2140/akt.2022.7.191","DOIUrl":"https://doi.org/10.2140/akt.2022.7.191","url":null,"abstract":". The present paper is a continuation of earlier work by Gunnar Carlsson and the first author on a motivic variant of the classical Becker-Gottlieb transfer and an additivity theorem for such a transfer by the present authors. Here, we establish a motivic variant of the classical Segal-Becker theorem relating the classifying space of a 1-dimensional torus with the spectrum defining algebraic K-theory.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-06-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46878439","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A characteristic property of cohomology with compact support is the long exact sequence that connects the compactly supported cohomology groups of a space, an open subspace and its complement. Given an arbitrary cohomology theory of algebraic varieties, one can ask whether a compactly supported version exists, satisfying such a long exact sequence. This is the case whenever the cohomology theory satisfies descent for abstract blowups (also known as proper cdh descent). We make this precise by proving an equivalence between certain categories of hypersheaves. We show how several classical and non-trivial results, such as the existence of a unique weight filtration on cohomology with compact support, can be derived from this theorem.
{"title":"A descent principle for compactly supported extensions of functors","authors":"Josefien Kuijper","doi":"10.2140/akt.2023.8.489","DOIUrl":"https://doi.org/10.2140/akt.2023.8.489","url":null,"abstract":"A characteristic property of cohomology with compact support is the long exact sequence that connects the compactly supported cohomology groups of a space, an open subspace and its complement. Given an arbitrary cohomology theory of algebraic varieties, one can ask whether a compactly supported version exists, satisfying such a long exact sequence. This is the case whenever the cohomology theory satisfies descent for abstract blowups (also known as proper cdh descent). We make this precise by proving an equivalence between certain categories of hypersheaves. We show how several classical and non-trivial results, such as the existence of a unique weight filtration on cohomology with compact support, can be derived from this theorem.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-04-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46071752","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We study the noncommutative geometry of algebras of Lipschitz continuous and H"older continuous functions where non-classical and novel differential geometric invariants arise. Indeed, we introduce a new class of Hochschild and cyclic cohomology classes that pair non-trivially with higher algebraic $K$-theory yet vanish when restricted to the algebra of smooth functions. Said cohomology classes provide additional methods to extract numerical invariants from Connes-Karoubi's relative sequence in $K$-theory.
{"title":"Exotic cyclic cohomology classes and Lipschitz algebras","authors":"M. Goffeng, R. Nest","doi":"10.2140/akt.2023.8.221","DOIUrl":"https://doi.org/10.2140/akt.2023.8.221","url":null,"abstract":"We study the noncommutative geometry of algebras of Lipschitz continuous and H\"older continuous functions where non-classical and novel differential geometric invariants arise. Indeed, we introduce a new class of Hochschild and cyclic cohomology classes that pair non-trivially with higher algebraic $K$-theory yet vanish when restricted to the algebra of smooth functions. Said cohomology classes provide additional methods to extract numerical invariants from Connes-Karoubi's relative sequence in $K$-theory.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47702706","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Let A be a C-algebra with an action of a finite group G, let $natural$ be a 2-cocycle on $G$ and consider the twisted crossed product $A rtimes C [G,natural]$. We determine the Hochschild homology of $A rtimes C [G,natural]$ for two classes of algebras A: - rings of regular functions on nonsingular affine varieties, - graded Hecke algebras. The results are achieved via algebraic families of (virtual) representations and include a description of the Hochschild homology as module over the centre of $A rtimes C [G,natural]$. This paper prepares for a computation of the Hochschild homology of the Hecke algebra of a reductive p-adic group.
设A是一个作用于有限群G的C代数，设$natural$是$G$上的一个2环，并考虑其扭曲叉积$A r乘以C [G，natural]$。我们确定了两类代数A:非奇异仿射变异-分阶Hecke代数上正则函数环的Hochschild同调。结果是通过(虚拟)表示的代数族实现的，并包括Hochschild同调的描述，作为$ a rtimes C [G，natural]$中心上的模。本文准备了一个约化p进群的Hecke代数的Hochschild同调的计算。
{"title":"Hochschild homology of twisted crossed products and twisted graded Hecke algebras","authors":"M. Solleveld","doi":"10.2140/akt.2023.8.81","DOIUrl":"https://doi.org/10.2140/akt.2023.8.81","url":null,"abstract":"Let A be a C-algebra with an action of a finite group G, let $natural$ be a 2-cocycle on $G$ and consider the twisted crossed product $A rtimes C [G,natural]$. We determine the Hochschild homology of $A rtimes C [G,natural]$ for two classes of algebras A: - rings of regular functions on nonsingular affine varieties, - graded Hecke algebras. The results are achieved via algebraic families of (virtual) representations and include a description of the Hochschild homology as module over the centre of $A rtimes C [G,natural]$. This paper prepares for a computation of the Hochschild homology of the Hecke algebra of a reductive p-adic group.","PeriodicalId":42182,"journal":{"name":"Annals of K-Theory","volume":null,"pages":null},"PeriodicalIF":0.6,"publicationDate":"2022-01-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48266979","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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