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Rasool Hafezi

Publications and source records attributed to Rasool Hafezi.

35 records · Page 2Linked to original sources

On the Monomorphism Category of $n$-Cluster Tilting Subcategories

Let $\mathcal{M}$ be an $n$-cluster tilting subcategory of ${\rm mod}\mbox{-}Λ$, where $Λ$ is an artin algebra. Let $\mathcal{S}(\mathcal{M})$ denotes the full subcategory of $\mathcal{S}(Λ)$, the submodule category of $Λ$, consisting of all monomorphisms in $\mathcal{M}$. We construct two functors from $\mathcal{S}(\mathcal{M})$ to ${\rm mod}\mbox{-}\underline{\mathcal{M}}$, the category of finitely presented (coherent) additive contravariant functors on the stable category of $\mathcal{M}$. We show that these functors are full, dense and objective. So they induce equivalences from the quotient categories of the submodule category of $\mathcal{M}$ modulo their respective kernels. Moreover, they are related by a syzygy functor on the stable category of ${\rm mod}\mbox{-}\underline{\mathcal{M}}$. These functors can be considered as a higher version of the two functors studied by Ringel and Zhang [RZ] in the case $Λ=k[x]/{\langle x^n \rangle}$ and generalized later by Eiríksson [E] to self-injective artin algebras. Several applications will be provided.

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Relative Singularity categories and singular equivalences

Let $R$ be a right notherian ring. We introduce the concept of relative singularity category $Δ_{\mathcal{X}}(R)$ of $R$ with respect to a contravariantly finite subcategory $\mathcal{X}$ of $\rm{mod}\mbox{-}R.$ Along with some finiteness conditions on $\mathcal{X}$, we prove that $Δ_{\mathcal{X}}(R)$ is triangle equivalent to a subcategory of the homotopy category $\mathbb{K}_{\rm{ac}}(\mathcal{X})$ of exact complexes over $\mathcal{X}$. As an application, a new description of the classical singularity category $\mathbb{D}_{\rm{sg}}(R)$ is given. The relative singularity categories are applied to lift a stable equivalence between two suitable subcategories of the module categories of two given right notherian ring to get a singular equivalence between the rings. In different types of rings, including path rings, triangular matrix rings, trivial extension rings and tensor rings, we provide some consequences for their singularity categories.

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Different exact structures on the monomorphism categories

Let $\mathcal{X}$ be a resolving and contravariantly finite subcategory of $\rm{mod}\mbox{-}Λ$, the category of finitely generated right $Λ$-modules. We associate to $\mathcal{X}$ the subcategory $\mathcal{S}_{\mathcal{X}}(Λ)$ of the morphism category $\rm{H}(Λ)$ consisting of all monomorphisms $(A\stackrel{f}\rightarrow B)$ with $A, B$ and $\rm{Cok} \ f$ in $\mathcal{\mathcal{X}}$. Since $\mathcal{S}_{\mathcal{X}}(Λ)$ is closed under extensions then it inherits naturally an exact structure from $\rm{H}(Λ)$. We will define two other different exact structures else than the canonical one on $\mathcal{S}_{\mathcal{X}}(Λ)$, and the indecomposable projective (resp. injective) objects in the corresponding exact categories completely classified. Enhancing $\mathcal{S}_{\mathcal{X}}(Λ)$ with the new exact structure provides a framework to construct a triangle functor. Let $\rm{mod}\mbox{-}\underline{\mathcal{X}}$ denote the category of finitely presented functors over the stable category $\underline{\mathcal{X}}$. We then use the triangle functor to show a triangle equivalence between the bounded derived category $\mathbb{D}^{\rm{b}}(\rm{mod}\mbox{-}\underline{\mathcal{X}})$ and a Verdier quotient of the bounded derived category of the associated exact category on $\mathcal{S}_{\mathcal{X}}(Λ)$. Similar consideration is also given for the singularity category of $\rm{mod}\mbox{-}\underline{\mathcal{X}}$.

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$n\mathbb{Z}$-Gorenstein cluster tilting subcategories

Let $Λ$ be an artin algebra. In this paper, the notion of $n\mathbb{Z}$-Gorenstein cluster tilting subcategories will be introduced. It is shown that every $n\mathbb{Z}$-cluster tilting subcategory of ${\rm{mod}}{\mbox{-}}Λ$ is $n\mathbb{Z}$-Gorenstein if and only if $Λ$ is an Iwanaga-Gorenstein algebra. Moreover, it will be shown that an $n\mathbb{Z}$-Gorenstein cluster tilting subcategory of ${\rm{mod}}{\mbox{-}}Λ$ is an $n\mathbb{Z}$-cluster tilting subcategory of the exact category ${\rm{Gprj}}{\mbox{-}}Λ$, the subcategory of all Gorenstein projective objects of ${\rm{mod}}{\mbox{-}}Λ$. Some basic properties of $n\mathbb{Z}$-Gorenstein cluster tilting subcategories will be studied. In particular, we show that they are $n$-resolving, a higher version of resolving subcategories.

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A ring theoretic approach to the finite representation type

An Artin algebra $Λ$ is said to be of finite Cohen-Macaulay type, $\rm{CM}$-finite for short, if the full subcategory $\rm{Gprj}\mbox{-} Λ$ of finitely generated Gorenstein projective $Λ$-modules is of finite representation type. If $Λ$ is a $\rm{CM}$-finite algebra, then we denote by $\rm{Aus}(\underline{\rm{Gprj}}\mbox{-} Λ)$ the stable Cohen-Macaulay Auslander algebra, i.e. $\rm{\underline{End}}_Λ(G)$, where $G $ is a basic representation generator of $\rm{Gprj}\mbox{-}Λ$. In this paper, we will explain how by defining an equivalence relation on the elements of algebra $\rm{Aus}(\underline{\rm{Gprj}}\mbox{-} Λ)$ can be used to give a characterization for $\rm{Aus}(\underline{\rm{Gprj}}\mbox{-} Λ)$ to be of finite representation type, or equivalently, the $\rm{CM}$-finiteness of the algebra of $2 \times 2$ lower triangular matrices over $Λ,$ where $Λ$ is a $\rm{CM}$-finite Artin algebra over an algebraic closed filed. Then, by presenting some examples we will show how our results work.

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Auslander's Formula for contravariantly finite subcategories

A relative version of Auslander's formula with respect to a contravariantly finite subcategory will be given. Dual version will be treated. Several examples and applications will be provided. In particular, we show that under certain circumstances, if relative Auslander algebras of artin algebras $Λ$ and $Λ'$ are Morita equivalent, then $Λ$ and $Λ'$ are also Morita equivalent.

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Auslander-Reiten duality for subcategories

Auslander-Reiten duality for module categories is generalized to some sufficiently nice subcategories. In particular, our consideration works for $\mathcal{P}^{<\infty}(Λ)$, the subcategory consisting of finitely generated modules with finite projective dimension over an artin algebra $Λ$, and also, the subcategory of Gorenstein projectove modules of $\rm{mod}\mbox{-}Λ$, denoted by $\rm{Gprj}\mbox{-}Λ$. In this paper, we give a method to compute the Auslander-Reiten translation in $\mathcal{P}^{<\infty}(Λ)$ whenever $Λ$ is a $1$-Gorenstein algebra. In addition, we characterize when the Auslander-Reiten translation in $\rm{Gprj}\mbox{-}Λ$ is the first syzygy and provide many algebras having such property.

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On relative Auslander algebras

Relative Auslander algebras were introduced and studied by Beligiannis. In this paper, we apply intermediate extension functors associated to certain recollements of functor categories to study them. In particular, we study the existence of tilting-cotilting modules over such algebras. As a consequence, it will be shown that two Gorenstein algebras of G-dimension 1 being of finite Cohen-Macaulay type are Morita equivalent if and only if their Cohen-Macaulay Auslander algebras are Morita equivalent.

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Auslander's Formula: Variations and Applications

According to the Auslander's formula one way of studying an abelian category ${\mathcal{C}}$ is to study ${\rm mod}\mbox{-}{\mathcal{C}}$, that has nicer homological properties than ${\mathcal{C}}$, and then translate the results back to ${\mathcal{C}}$. Recently Krause gave a derived version of this formula and thus renewed the subject. This paper contains a detailed study of various versions of Auslander formula including the versions for all modules and for unbounded derived categories. We apply them to include some results concerning recollements of triangulated categories.

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Minimal Injective Resolutions and Auslander-Gorenstein Property for Path Algebras

Let $R$ be a ring and $\mathcal{Q}$ be a finite and acyclic quiver. We present an explicit formula for the injective envelopes and projective precovers in the category $\rm{Rep} (\mathcal{Q} ,R)$ of representations of $\mathcal{Q}$ by left $R$-modules. We also extend our formula to all terms of the minimal injective resolution of $R\mathcal{Q}$. Using such descriptions, we study the Auslander-Gorenstein property of path algebras. In particular, we prove that the path algebra $R\mathcal{Q}$ is $k$-Gorenstein if and only if $\mathcal{Q}=\overrightarrow{A_{n}}$ and $R$ is a $k$-Gorenstein ring, where $n$ is the number of vertices of $\mathcal{Q}$.

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Derived equivalences of functor categories

Let $\Mod \CS$ denote the category of $\CS$-modules, where $\CS$ is a small category. In the first part of this paper, we provide a version of Rickard's theorem on derived equivalence of rings for $\Mod \CS$. This will have several interesting applications. In the second part, we apply our techniques to get some interesting recollements of derived categories in different levels. We specialize our results to path rings as well as graded rings.

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Homotopy category of N-complexes of projective modules

In this paper, we show that the homotopy category of N-complexes of projective R-modules is triangle equivalent to the homotopy category of projective T_{N-1}(R)- modules where T_{N-1}(R) is the ring of triangular matrices of order N-1 with entries in R. We also define the notions of N-singularity category and N-totally acyclic complexes. We show that the category of N-totally acyclic complexes of finitely generated projective R-modules embeds in the N-singularity category, which is a result analogous to the case of ordinary chain complexes.

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Recollements of Cohen-Macaulay Auslander algebras and Gorenstein derived categories

Let $A$, $B$ and $C$ be associative rings with identity. Using a result of Koenig we show that if we have a $\mathbb{D}^{\rm{b}}({\rm{mod\mbox{-}}} )$ level recollement, writing $A$ in terms of $B$ and $C$, then we get a $\mathbb{D}^-({\rm{Mod\mbox{-}}} )$ level recollement of certain functor categories, induces from the module categories of $A$, $B$ and $C$. As an application, we generalise the main theorem of Pan [Sh. Pan, Derived equivalences for Cohen-Macaulay Auslander algebras, J. Pure Appl. Algebra, 216 (2012), 355-363] in terms of recollements of Gorenstein artin algebras. Moreover, we show that being Gorenstein as well as being of finite Cohen-Macaulay type, are invariants with respect to $\mathbb{D}^{\rm{b}}_{{{\mathcal{G}p}}}({\rm{mod\mbox{-}}})$ level recollements of virtually Gorenstein algebras, where $\mathbb{D}^{\rm{b}}_{{{\mathcal{G}p}}}$ denotes the Gorenstein derived category.

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Gorenstein Projective Modules Over Triangular Matrix Rings

We study totally acyclic complexes of projective modules over triangular matrix rings and then use it to classify Gorenstein projective modules over such rings. We also use this classification to obtain some information concerning Cohen-Macaulay finite and virtually Gorenstein triangular matrix artin algebras.

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Homotopy category of projective complexes and complexes of Gorenstein projective modules

Let $R$ be a ring with identity and $\C(R)$ denote the category of complexes of $R$-modules. In this paper we study the homotopy categories arising from projective (resp. injective) complexes as well as Gorenstein projective (resp. Gorenstein injective) modules. We show that the homotopy category of projective complexes over $R$, denoted $\KPC$, is always well generated and is compactly generated provided $\KPR$ is so. Based on this result, it will be proved that the class of Gorenstein projective complexes is precovering, whenever $R$ is a commutative noetherian ring of finite Krull dimension. Furthermore, it turns out that over such rings the inclusion functor $ι: \K(\RGPrj)\hookrightarrow \KR$ has a right adjoint $ι_ρ$, where $\K(\RGPrj)$ is the homotopy category of Gorenstein projective $R$ modules. Similar, or rather dual, results for the injective (resp. Gorenstein injective) complexes will be provided. If $R$ has a dualising complex, a triangle-equivalence between homotopy categories of projective and of injective complexes will be provided. As an application, we obtain an equivalence between the triangulated categories $\K(\RGPrj)$ and $\K(\RGInj)$, that restricts to an equivalence between $\KPR$ and $\KIR$, whenever $R$ is commutative, noetherian and admits a dualising complex.

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On the Dimensions of Path Algebras

In this paper we study the representation dimension as well as the derived dimension of the path algebra of an artin algebra over a finite and acyclic quiver.

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On the derived dimension of abelian categories

We give an upper bound on the dimension of the bounded derived category of an abelian category. We show that if $\CX$ is a sufficiently nice subcategory of an abelian category, then derived dimension of $\CA$ is at most $\CX$-dim$\CA$, provided $\CX$-dim$\CA$ is greater than one. We provide some applications.

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