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Amnon Yekutieli

Publications and source records attributed to Amnon Yekutieli.

At least 19 recordsLinked to original sources
arXiv2026

The Simplicial Cylinder DG Ring

The Keller cylinder DG ring encodes homotopies between DG ring homomorphisms $f_0, f_1 : A \to B$. Recently we discovered the higher cylinder DG rings $Cyl_q(B)$, which assemble into the simplicial cylinder DG ring $Cyl(B)$. For $q=1$ this recovers Keller's original construction. The sets $SHom_q(A,B)$ of DG ring homomorphisms $A \to Cyl_q(B)$ form the simplicial Hom set $SHom(A,B)$. Our main result is that when $A$ is a semi-free DG ring, the simplicial set $SHom(A,B)$ is a Kan complex. We prove several results about the fundamental groupoid $SHom_{\leq 1}(A,B)$, including invariance under quasi-isomorphism $B' \to B$, and that the automorphism groups are abelian. We also indicate some applications of this work.

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arXiv2024

Derived Complete Complexes at Weakly Proregular Ideals

Weak proregularity of an ideal in a commutative ring is a subtle generalization of the noetherian property of the ring. Weak proregularity is of special importance for the study of derived completion, and it occurs quite often in non-noetherian rings arising in Hochschild and prismatic cohomologies. This paper is about several related topics: adically flat modules, recognizing derived complete complexes, the structure of the category of derived complete complexes, and a derived complete Nakayama theorem - all with respect to a weakly proregular ideal; and the preservation of weak proregularity under completion of the ring.

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arXiv2024

On Adically Complete D-Modules in Characteristic Zero

Let (X, O_X) be an algebraic manifold in characteristic 0, or an analytic manifold over \C. A standard theorem says that a left D_X-module M, which is coherent as an O_X-module, is locally free. This theorem has a generalization to the adically complete algebraic setting, in a paper by Ogus from 1973. In the present paper we take a new look at the work of Ogus. We provide a detailed proof of the theorem on D-modules, and extend it to the non-noetherian setting. We also give another proof of an interesting result of Ogus about adically complete modules (slightly extended). In the Appendix we discuss a related error in a book by Bjork.

math.AG↗
arXiv2024

Rigid Dualizing Complexes over Commutative Rings and their Functorial Properties

In this paper we treat Grothendieck Duality for noetherian rings via rigid dualizing complexes. In particular, we prove that every ring, essentially finite type over a regular base ring, has a unique rigid dualizing complex. The rigid dualizing complexes have strong functorial properties, allowing us to construct the twisted induction pseudofunctor, which is our ring-theoretic version of the twisted inverse pseudofunctor $f^{!}$. This is the first article of a bigger project, whose final goal is establishing Grothendieck Duality, including global duality for proper maps, for Deligne-Mumford stacks.

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arXiv2023

Quasi-Isomorphisms of Commutative DG Rings and Divided Power Structures

We prove that a quasi-isomorphism $f : A \to B$ between commutative DG rings, where $B$ admits a divided power structure, can be factored as $f = \tilde{f} \circ e$, where $e : A \to \tilde{B}$ is a split injective quasi-isomorphism, and $\tilde{f} : \tilde{B} \to B$ is a surjective quasi-isomorphism. This result is used in our work on a DG approach to the cotangent complex, and our work on the derived category of commutative DG rings.

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arXiv2023

Improved Kunneth Tricks

The Kunneth trick is a formula for the top cohomology of the derived tensor product of two complexes of modules over a ring. In this note we present two improvements of this formula. The first improved Kunneth trick is a formula for the top cohomology of the plain tensor product of two DG modules over a nonpositive DG ring. The second trick handles the derived tensor product of two DG modules over a nonpositive DG ring. The proofs are elementary.

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arXiv2022

Rings of Bounded Continuous Functions

We examine several classical concepts from topology and functional analysis, using methods of commutative algebra. We show that these various concepts are all controlled by BC R-rings and their maximal spectra. A BC R-ring is a ring A that is isomorphic to the ring of bounded continuous R-valued functions on some compact topological space X. These rings are not topologized. We prove that the category of BC R-rings is dual to the category of compact topological spaces. Next we prove that for every topological space X the ring of bounded continuous functions on it is a BC R-ring. These theorems combined yield an algebraic construction of the Stone-Cech Compactification of an arbitrary topological space. There is a similar notion of BC C-ring. Every BC C-ring A has a canonical involution. The canonical hermitian subring of A is a BC R-ring, and this is an equivalence of categories from BC C-rings to BC R-rings. Let K be either R or C. We prove that a BC K-ring A has a canonical norm on it, making it into a Banach K-ring. We then prove that the forgetful functor is an equivalence from Banach^* K-rings (better known as commutative unital C^* K-algebras) to BC K-rings. The quasi-inverse of the forgetful functor endows a BC K-ring with its canonical norm, and the canonical involution when K = C. Stone topological spaces, also known as profinite topological spaces, are traditionally related to boolean rings - this is Stone Duality. We give a BC ring characterization of Stone spaces. From that we obtain a very easy proof of the fact that the Stone-Cech Compactification of a discrete space is a Stone space. Most of the results in this paper are not new. However, most of our proofs seem to be new - and our methods could potentially lead to genuine progress related to these classical topics.

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arXiv2022

Pythagorean Triples, Complex Numbers, Abelian Groups and Prime Numbers

It is well-known that pythagorean triples can be represented by points of the unit circle with rational coordinates. These points form an abelian group, and we describe its structure. This structural description yields, almost immediately, an enumeration of the normalized pythagorean triples with a given hypotenuse, and also to an effective method for producing all such triples. This effective method seems to be new. This paper is intended for the general mathematical audience, including undergraduate mathematics students, and therefore it contains plenty of background material, some history and several examples and exercises.

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arXiv2021

Weak Proregularity, Derived Completion, Adic Flatness, and Prisms

This paper has two parts. In the first part we recall the important role that weak proregularity of an ideal in a commutative ring has in derived completion and in adic flatness. We also introduce the new concepts of idealistic and sequential derived completion, and prove a few results about them, including the fact that these two concepts agree iff the ideal is weakly proregular. In the second part we study the local nature of weak proregularity, and its behavior w.r.t. ring quotients. These results allow us to prove that weak proregularity occurs in the context of bounded prisms, in the sense of Bhatt and Scholze. We anticipate that the concept of weak proregularity will help simplify and improve some of the more technical aspects of the groundbreaking theory of perfectoid rings and prisms (that has transformed arithmetic geometry in recent years).

math.AC↗
arXiv2021

Rigidity, Residues and Duality: Overview and Recent Progress

In this article we explain the theory of rigid residue complexes in commutative algebra and algebraic geometry, summarizing the background, recent results and anticipated future results. Unlike all previous approaches to Grothendiec Duality, the rigid approach concentrates on the construction of rigid residue complexes over rings, and their intricate yet robust properties. The geometrization, i.e. the passage to rigid residue complexes on schemes and Deligne-Mumford (DM) stacks, by gluing, is fairly easy. In the geometric part of the theory, the main results are the Rigid Residue Theorem and the Rigid Duality Theorem for proper maps between schemes, and for tame proper maps between DM stacks.

math.AG↗
arXiv2020

Rigid Dualizing Complexes on Schemes

In this paper we present a new approach to Grothendieck duality on schemes. Our approach is based on the idea of rigid dualizing complexes, which was introduced by Van den Bergh in the context of noncommutative algebraic geometry. We obtain most of the important features of Grothendieck duality, yet manage to avoid lengthy and difficult compatibility verifications. Our results apply to finite type schemes over a regular noetherian finite dimensional base ring, and hence are suitable for arithmetic geometry.

math.AG↗
arXiv2019

Derived Categories

This is the fourth (and last) prepublication version of a book on derived categories, that will be published by Cambridge University Press. The purpose of the book is to provide solid foundations for the theory of derived categories, and to present several applications of this theory in commutative and noncommutative algebra. The emphasis is on constructions and examples, rather than on axiomatics. Here are the topics covered in the book: - A review of standard facts on abelian categories. - Differential graded algebra (DG rings, DG modules, DG categories and DG functors). - Triangulated categories and triangulated functors between them. How they arise from the DG background. The homotopy category K(A,M) of DG A-modules in M. - Localization of categories. The derived category D(A,M), which is the localization of K(A,M) with respect to the quasi-isomorphisms. - Left and right derived functors of a triangulated functor. - K-injective, K-projective and K-flat DG modules. Their roles, and their existence in several important algebraic situations. - Dualizing and residue complexes over commutative noetherian rings, including Van den Bergh rigidity. - Perfect DG modules and tilting DG bimodules over NC (noncommutative) DG rings. - NC connected graded rings, including Artin-Schelter regular rings. Derived torsion for NC connected graded rings, its relation to the chi condition of Artin-Zhang, and the NC MGM Equivalence. Balanced dualizing complexes, their uniqueness, existence and trace functoriality. - NC rigid dualizing complexes, following Van den Bergh. The uniqueness and existence of these complexes, a few examples, and their relation to Calabi-Yau rings. Readers of this preview version are urged to write to the author with any comments regarding errors, suggestions or questions.

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arXiv2018

Weak Proregularity, Weak Stability, and the Noncommutative MGM Equivalence

Let A be a commutative ring, and let \a = \frak{a} be a finitely generated ideal in it. It is known that a necessary and sufficient condition for the derived \a-torsion and \a-adic completion functors to be nicely behaved is the weak proregularity of \a. In particular, the MGM Equivalence holds. Because weak proregularity is defined in terms of elements of the ring (specifically, it involves limits of Koszul complexes), it is not suitable for noncommutative ring theory. In this paper we introduce a new condition on a torsion class T in a module category: weak stability. Our first main theorem is that in the commutative case, the ideal \a is weakly proregular if and only if the corresponding torsion class T_{\a} is weakly stable. We then study weak stability of torsion classes in module categories over noncommutative rings. There are three main theorems in this context: - For a torsion class T that is weakly stable, quasi-compact and finite dimensional, the right derived torsion functor is isomorphic to a left derived tensor functor. - The Noncommutative MGM Equivalence, under the same assumptions on T. - A theorem about symmetric derived torsion for complexes of bimodules. This last theorem is a generalization of a result of Van den Bergh from 1997, and corrects an error in a paper of Yekutieli-Zhang from 2003.

math.RA↗
arXiv2017

Flatness and Completion Revisited

We continue investigating the interaction between flatness and $\mathfrak{a}$-adic completion for infinitely generated modules over a commutative ring $A$. We introduce the concept of $\mathfrak{a}$-adic flatness, which is weaker than flatness. We prove that $\mathfrak{a}$-adic flatness is preserved under completion when the ideal $\mathfrak{a}$ is weakly proregular. We also prove that when $A$ is noetherian, $\mathfrak{a}$-adic flatness coincides with flatness (for complete modules). An example is worked out of a non-noetherian ring $A$, with a weakly proregular ideal $\mathfrak{a}$, for which the completion $\hat{A}$ is not flat. We also study $\mathfrak{a}$-adic systems, and prove that if the ideal $\mathfrak{a}$ is finitely generated, then the limit of any $\mathfrak{a}$-adic system is a complete module.

math.AC↗
arXiv2016

The Derived Category of Sheaves of Commutative DG Rings (Preview)

In this short paper we outline (mostly without proofs) our new approach to the derived category of sheaves of commutative DG rings. The proofs will appear in a subsequent paper. Among other things, we explain how to form the derived intersection of two closed subschemes inside a given algebraic scheme X, without recourse to simplicial or higher homotopical methods, and without any global assumptions on X.

math.AG↗
arXiv2016

Duality and Tilting for Commutative DG Rings

We consider commutative DG rings (better known as nonpositive strongly commutative associative unital DG algebras). For such a DG ring $A$ we define the notions of perfect, tilting, dualizing, Cohen-Macaulay and rigid DG $A$-modules. Geometrically perfect DG modules are defined by a local condition on $\operatorname{Spec} \bar{A}$, where $\bar{A} := \operatorname{Spec} \, \operatorname{H}^0(A)$. Algebraically perfect DG modules are those that can be obtained from $A$ by finitely many shifts, direct summands and cones. Tilting DG modules are those that have inverses w.r.t. the derived tensor product; their isomorphism classes form the derived Picard group $\operatorname{DPic}(A)$. Dualizing DG modules are a generalization of Grothendieck's original definition (and here $A$ has to be cohomologically pseudo-noetherian). Cohen-Macaulay DG modules are the duals (w.r.t. a given dualizing DG module) of finite $\bar{A}$-modules. Rigid DG $A$-modules, relative to a commutative base ring $K$, are defined using the squaring operation, and this is a generalization of Van den Bergh's original definition. The techniques we use are the standard ones of derived categories, with a few improvements. We introduce a new method for studying DG $A$-modules: Cech resolutions of DG $A$-modules corresponding to open coverings of $\operatorname{Spec} \bar{A}$. Here are some of the new results obtained in this paper:... [truncated] The functorial properties of Cohen-Macaulay DG modules that we establish here are needed for our work on rigid dualizing complexes over commutative rings, schemes and Deligne-Mumford stacks. We pose several conjectures regarding existence and uniqueness of rigid DG modules over commutative DG rings.

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arXiv2015

The Squaring Operation for Commutative DG Rings

Let A -> B be a homomorphism of commutative rings. The squaring operation is a functor Sq_{B/A} from the derived category D(B) of complexes B-modules into itself. The squaring operation is needed for the definition of rigid complexes (in the sense of Van den Bergh), that in turn leads to a new approach to Grothendieck duality for rings, schemes and even DM stacks. In our paper with J.J. Zhang from 2008 we introduced the squaring operation, and explored some of its properties. Unfortunately some of the proofs in that paper had severe gaps in them. In the present paper we reproduce the construction of the squaring operation. This is done in a somewhat more general context than in the first paper: here we consider a homomorphism A -> B of commutative DG rings. Our first main result is that the square Sq_{B/A}(M) of a DG B-module M is independent of the resolutions used to present it. Our second main result is on the trace functoriality of the squaring operation. We give precise statements and complete correct proofs. In a subsequent paper we will reproduce the remaining parts of the 2008 paper that require fixing. This will allow us to proceed with the other papers, mentioned in the bibliography, on the rigid approach to Grothendieck duality. The proofs of the main results require a substantial amount of foundational work on commutative and noncommutative DG rings, including a study of semi-free DG rings, their lifting properties, and their homotopies. This part of the paper could be of independent interest.

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