arXiv subjects

Jeremy Russell

Publications and source records attributed to Jeremy Russell.

7 recordsLinked to original sources

arXiv2017

Injective stabilization of additive functors. III. Asymptotic stabilization of the tensor product

The injective stabilization of the tensor product is subjected to an iterative procedure that utilizes its bifunctor property. The limit of this procedure, called the asymptotic stabilization of the tensor product, provides a homological counterpart of Buchweitz's asymptotic construction of stable cohomology. The resulting connected sequence of functors is isomorphic to Triulzi's $J$-completion of the Tor functor. A comparison map from Vogel homology to the asymptotic stabilization of the tensor product is constructed and shown to be always epic. The category of finitely presented functors is shown to be complete and (co)complete. As a consequence, the inert injective stabilization of the tensor product with fixed variable a finitely generated module over an artin algebra is shown to be finitely presented. A description of its defect and all right-derived functors is given. A surprising connection with Buchweitz cohomology based on injectives is established

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arXiv2016

Injective stabilization of additive functors. I. Preliminaries

This paper is the first one in a series of three dealing with the concept of injective stabilization of the tensor product and its applications. Its primary goal is to collect known facts and establish a basic operational calculus that will be used in the subsequent parts. This is done in greater generality than is necessary for the stated goal. Several results of independent interest are also established. They include, among other things, connections with satellites, an explicit construction of the stabilization of a finitely presented functor, various exactness properties of the injectively stable functors, a construction, from a functor and a short exact sequence, of a doubly-infinite exact sequence by splicing the injective stabilization of the functor and its derived functors. When specialized to the tensor product with a finitely presented module, the injective stabilization with coefficients in the ring is isomorphic to the 1-torsion functor. The Auslander-Reiten formula is extended to a more general formula, which holds for arbitrary (i.e., not necessarily finite) modules over arbitrary associative rings with identity. Weakening of the assumptions in the theorems of Eilenberg and Watts leads to characterizations of the requisite zeroth derived functors. The subsequent papers, provide applications of the developed techniques. Part~II deals with new notions of torsion module and cotorsion module of a module. This is done for arbitrary modules over arbitrary rings. Part~III introduces a new concept, called the asymptotic stabilization of the tensor product. The result is closely related to different variants of stable homology (these are generalizations of Tate homology to arbitrary rings). A comparison transformation from Vogel homology to the asymptotic stabilization of the tensor product is constructed and shown to be epic.

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arXiv2016

Injective stabilization of additive functors. II. (Co)torsion and the Auslander-Gruson-Jensen functor

The formalism of injective stabilization of additive functors is used to define a new notion of the torsion submodule of a module. It applies to arbitrary modules over arbitrary rings. For arbitrary modules over commutative domains it coincides with the classical torsion, and for finitely presented modules over arbitrary rings it coincides with the Bass torsion. A formally dual approach -- based on projective stabilization -- gives rise to a new concept: the cotorsion quotient module of a module. This is done in complete generality -- the new concept is defined for any module over any ring. Unlike torsion, cotorsion does not have classical prototypes. General properties of these constructs are established. It is shown that the Auslander-Gruson-Jensen functor applied to the cotorsion functor returns the torsion functor. As a consequence, a ring is one-sided absolutely pure if and only if each pure injective on the other side is cotorsion-free. If the injective envelope of the ring is finitely presented, then the right adjoint of the Auslander-Gruson-Jensen functor applied to the torsion functor returns the cotorsion functor. This correspondence establishes a duality between torsion and cotorsion over such rings. In particular, this duality applies to artin algebras. It is also shown that, over any ring, the character module of the torsion of a module is isomorphic to the cotorsion of the character module of the module. Under various finiteness conditions on the injective envelope of the ring, the derived functors of torsion and cotorsion are computed.

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arXiv2016

Derived Recollements and Generalised AR Formulas

The Defect Recollement, Restriction Recollement, Auslander-Gruson-Jensen Recollement, and others, are shown to be instances of a general construction using derived functors and methods from stable module theory. The right derived functors $\textsf{W}_k:=R_k(\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} )^*$ are computed and it is shown that the functor $\textsf{W}_2:=R_2(\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} )^*$ is right exact and restricts to a duality $\textsf{W}$ of the defect zero functors. The duality $\textsf{W}$ satisfies two identities which we call the Generalised Auslander-Reiten formulas. We show that $\textsf{W}$ restricts to the generalised Auslander-Bridger transpose and show that the Generalised Auslander-Reiten formulas reduce to the well-known Auslander-Reiten formulas.

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arXiv2016

The Auslander-Gruson-Jensen Recollement

For any ring $R$, the Auslander-Gruson-Jensen functor is the exact contravariant functor $$\textsf{D}_A:\textsf{fp}(\textsf{Mod}(R),\textsf{Ab})\longrightarrow(\textsf{mod}(R^{op}),\textsf{Ab})$$ sending representable functors $(X,\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} )$ to tensor functors $X\otimes\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} $. We show that this functor admits a fully faithful left adjoint $\textsf{D}_L$ and a fully faithful right adjoint $\textsf{D}_R$. The left adjoint $$\textsf{D}_L\:(\textsf{mod}(R^{op}),\textsf{Ab})\longrightarrow \textsf{fp}(\textsf{Mod}(R),\textsf{Ab})$$ induces an equivalence of categories $$\frac{\textsf{fp}(\textsf{Mod}(R),\textsf{Ab})}{\{F\ |\ \textsf{D}_A F=0\}}\cong(\textsf{mod}(R^{op}),\textsf{Ab})^{op}$$ where $\{F \ |\ \textsf{D}_A F=0\}$ is the Serre subcategory of $\textsf{fp}(\textsf{Mod}(R),\textsf{Ab})$ consisting of all functors $F$ arising from pure exact sequences. As a result, the functor $\textsf{D}_A$ is seen to be a Serre localization functor. The right adjoint $$\textsf{D}_R:(\textsf{mod}(R^{op}),\textsf{Ab})\longrightarrow \textsf{fp}(\textsf{Mod}(R),\textsf{Ab})$$ together with $\textsf{D}_A$ restricts to the well known Auslander-Gruson-Jensen duality.

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arXiv2015

Horizontal Linkage of Coherent Functors

The satellite endofunctors are used to extend the definition of linkage of ideals to the linkage of totally finitely presented functors. The new notion for linkage works over a larger class of rings and is consistent with the functorial approach of encoding information about modules into the category of finitely presented functors. In the process of extending linkage, we recover the Auslander-Gruson-Jensen duality using injective resolutions of finitely presented functors. Using the satellite endofunctors we give general definitions of derived functors which do not require the existence of projective or injective objects. A general formula for calculating the defect of a totally finitely presented functor is given.

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arXiv2012

Applications of the Defect of a Finitely Presented Functor

For an abelian category $\mathcal{A}$, the defect sequence $$0\longrightarrow F_0\longrightarrow F\overset{\varphi}{\longrightarrow} \big(w(F),\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} \big)\longrightarrow F_1\longrightarrow 0$$ of a finitely presented functor is used to establish the CoYoneda Lemma. An application of this result is the $\textsf{fp}$-dual formula which states that for any covariant finitely presented functor $F$, $F^*\cong \big(\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} , w(F)\big)$. The defect sequence is shown to be isomorphic to both the double dual sequence $$0\longrightarrow \textsf{Ext}^1(\textsf{Tr} F,\textsf{Hom})\longrightarrow F\longrightarrow F^{**}\longrightarrow \textsf{Ext}^2(\textsf{Tr} F,\textsf{Hom})\longrightarrow 0$$ and the injective stabilization sequence $$0\longrightarrow \overline{F}\longrightarrow F\longrightarrow R^0F\longrightarrow \tilde F\longrightarrow 0$$ establishing the $\textsf{fp}$-injective stabilization formula $\overline{F}\cong \textsf{Ext}^1(\textsf{Tr} F,\textsf{Hom})$ for any finitely presented functor $F$. The injectives of $\textsf{fp}(\textsf{Mod}(R),\textsf{Ab})$ are used to compute the left derived functors $L^k(\hspace{0.05cm}\underline{\ \ }\hspace{0.1cm} )^*$. These functors are shown to detect certain short exact sequences.

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