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Tony J. Puthenpurakal

Publications and source records attributed to Tony J. Puthenpurakal.

At least 73 records · Page 4Linked to original sources
arXiv2023

Graded components of local cohomology modules of $\mathfrak{C}$-monomial ideals in characteristic zero

Let $A$ be a commutative Noetherian ring of characteristic zero and $R=A[X_1, \ldots, X_d]$ be a polynomial ring over $A$ with the standard $\mathbb{N}^d$-grading. Let $I\subseteq R$ be an ideal which can be generated by elements of the form $aU$ where $a \in A$ (possibly nonunit) and $U$ is a monomial in $X_i$'s. We call such an ideal as a `$\mathfrak{C}$-monomial ideal'. Local cohomology modules supported on monomial ideals gain a great deal of interest due to their applications in the context of toric varieties. It was observed that for $\underline{u} \in \mathbb{Z}^d$, their $\underline{u}^{th}$ components depend only on which coordinates of $\underline{u}$ are negative. In this article, we show that this statement holds true in our general setting, even for certain invariants of the components. We mainly focus on the Bass numbers, injective dimensions, dimensions, associated primes, Bernstein-type dimensions, and multiplicities of the components. Under the extra assumption that $A$ is regular, we describe the finiteness of Bass numbers of each component and bound its injective dimension by the dimension of its support. Finally, we present a structure theorem for the components when $A$ is the ring of formal power series in one variable over a characteristic zero field.

math.AC↗
arXiv2023

A sub-functor for Ext and Cohen-Macaulay associated graded modules with bounded multiplicity-II

Let $(A,\mathfrak{m})$ be a \CM\ local ring, then notion of $T$-split sequence was introduced in part-1 of this paper for $\mathfrak{m}$-adic filtration with the help of numerical function $e^T_A$. We have explored the relation between AR-sequences and $T$-split sequences. For a Gorenstein ring $(A,\mathfrak{m})$ we define a Hom-finite Krull-Remak-Schmidt category $\mathcal{D}_A$ as a quotient of the stable category $\CMS(A)$. This category preserves isomorphism, i.e. $M\cong N$ in $\mathcal{D}_A$ if and only if $M\cong N$ in $\CMS(A)$.This article has two objectives; first objective is to extend the notion of $T$-split sequence, and second objective is to explore function $e^T_A$ and $T$-split sequence. When $(A,\mathfrak{m})$ be an \anram\ \CM\ local ring and $I$ be an $\mathfrak{m}$-primary ideal then we extend the techniques in part-1 of this paper to the integral closure filtration with respect to $I$ and prove a version of Brauer-Thrall-II for a class of such rings.

math.AC↗
arXiv2023

On the Lowey length of modules of finite projective dimension-II

Let $(A,\mathfrak{m})$ be a Gorenstein local ring of dimension $d \geq 1$. Suppose there exists be a non-zero $A$ module $M$ of finite length and finite projective dimension such that $\ell\ell(M)$, the Lowey length of $M$, is equal to $λ(M)$, the length of $M$. Then we show that necessarily $A$ is at worst a hypersurface singularity. We also characterize Gorenstein local rings having a non-zero module $M$ of finite length and finite projective dimension with $\ell\ell(M) = λ(M)-1$.

math.AC↗
arXiv2023

Integrally closed $\mathfrak{m}$-primary ideals have extremal resolutions

We show that every integrally closed $\mathfrak{m}$-primary ideal $I$ in a commutative Noetherian local ring $(R,\mathfrak{m},k)$ has maximal complexity and curvature, i.e., $ {\rm cx}_R(I) = {\rm cx}_R(k) $ and $ {\rm curv}_R(I) = {\rm curv}_R(k) $. As a consequence, we characterize complete intersection local rings in terms of complexity, curvature and complete intersection dimension of such ideals. The analogous results on projective, injective and Gorenstein dimensions are known. However, we provide short proofs of these results as well.

math.AC↗
arXiv2023

On $p_g$-ideals in positive characteristic

Let $(A,\mathfrak{m})$ be an excellent normal domain of dimension two containing a field $k \cong A/\mathfrak{m}$. An $\mathfrak{m}$-primary ideal $I$ to be a $p_g$-ideal if the Rees algebra $A[It]$ is a Cohen-Macaulay normal domain. If $k$ is algebraically closed then Okuma, Watanabe and Yoshida proved that $A$ has $p_g$-ideals and furthermore product of two $p_g$-ideals is a $p_g$ ideal. In a previous paper we showed that if $k$ has characteristic zero then $A$ has $p_g$-ideals. In this paper we prove that if $k$ is perfect field of positive characteristic then also $A$ has $p_g$ ideals.

math.AC↗
arXiv2022

Itoh's conjecture for normal ideals

Let $(A,\mathfrak{m})$ be an analytically unramified Cohen-Macaulay local ring and let $\mathfrak{a}$ be an $\mathfrak{m}$-primary ideal in $A$. If $I$ is an ideal in $A$ then let $I^*$ be the integral closure of $I$ in $A$. Let $G_{\mathfrak{a}}(A)^* = \bigoplus_{n\geq 0 }(\mathfrak{a}^n)^*/(\mathfrak{a}^{n+1})^*$ be the associated graded ring of the integral closure filtration of $\mathfrak{a}$. Itoh conjectured that if $e_3^{\mathfrak{a}^*}(A) = 0$ and $A$ is Gorenstein then $G_{\mathfrak{a}}(A)^* $ is Cohen-Macaulay. In this paper we prove an important case of Itoh's conjecture: we show that if $A$ is Cohen-Macaulay and if $\mathfrak{a}$ is normal (i.e., $\mathfrak{a}^n$ is integrally closed for all $n \geq 1$) with $e_3^\mathfrak{a}(A) = 0$ then $G_\mathfrak{a}(A)$ is Cohen-Macaulay.

math.AC↗
arXiv2022

On $G(A)_\mathbb{Q}$ of rings of finite representation type

Let $(A,\mathfrak{m})$ be an excellent Henselian Cohen-Macaulay local ring of finite representation type. If the AR-quiver of $A$ is known then by a result of Auslander and Reiten one can explicity compute $G(A)$ the Grothendieck group of finitely generated $A$-modules. If the AR-quiver is not known then in this paper we give estimates of $G(A)_\mathbb{Q} = G(A)\otimes_\mathbb{Z} \mathbb{Q}$ when $k = A/\mathfrak{m}$ is perfect. As an application we prove that if $A$ is an excellent equi-characteristic Henselian Gornstein local ring of positive even dimension with $\text{char} \ A/\mathfrak{m} \neq 2,3,5$ (and $A/\mathfrak{m}$ perfect) then $G(A)_\mathbb{Q} \cong \mathbb{Q}$.

math.AC↗
arXiv2022

On a generalization of two results of Happel to commutative rings

In this paper we extend two results of Happel to commutative rings. Let $(A, \mathfrak{m})$ be a commutative Noetherian local ring. Let $D^b_f(mod \ A)$ be the bounded derived category of complexes of finitely generated modules over $A$ with finite length cohomology. We show $D^b_f( mod \ A)$ has Auslander-Reiten(AR)-triangles if and only if $A$ is regular. Let $K^b_f(proj \ A)$ be the homotopy category of finite complexes of finitely generated free $A$-modules with finite length cohomology. We show that if $A$ is complete and if $A$ is Gorenstein then $K^b_f( proj \ A)$ has AR triangles. Conversely we show that if $K^b_f(proj \ A)$ has AR triangles and if $A$ is Cohen-Macaulay or if $\dim A = 1$ then $A$ is Gorenstein.

math.AC↗
arXiv2022

On Coefficient ideals

Let $(A,\mathfrak{m})$ be a Cohen-Macaulay local ring of dimension $d \geq 2$ with infinite residue field and let $I$ be an $\mathfrak{m}$-primary ideal. For $0 \leq i \leq d$ let $I_i$ be the $i^{th}$-coefficient ideal of $I$. Also let $\widetilde{I} = I_d$ denote the Ratliff-Rush closure of $A$. Let $G = G_I(A)$ be the associated graded ring of $I$. We show that if $\dim H^j_{G_+}(G)^\vee \leq j -1$ for $1 \leq j \leq i \leq d-1$ then $(I^n)_{d-i} = \widetilde{I^n}$ for all $n \geq 1$. In particular if $G$ is generalized Cohen-Macaulay then $(I^n)_1 = \widetilde{I^n}$ for all $n \geq 1$. As a consequence we get that if $A$ is an analytically unramified domain with $G$ generalized Cohen-Macaulay, then the $S_2$-ification of the Rees algebra $ A[It]$ is $\bigoplus_{n \geq 0} \widetilde{I^n}$.

math.AC↗
arXiv2022

On associated graded modules of maximal Cohen-Macaulay modules over hypersurface rings

Let $A=Q/(f)$ where $(Q,\mathfrak{n})$ be a complete regular local ring of dimension $d+1$, $f\in \mathfrak{n}^i\setminus\mathfrak{n}^{i+1}$ for some $i\geq 2$ and $M$ an MCM $A-$module with $e(M)=μ(M)i(M)+1$ then we prove that depth $G(M)\geq d-1$. If $(A,\mathfrak{m})$ is a complete hypersurface ring of dimension $d$ with infinite residue field and $e(A)=3$, let $M$ be an MCM $A$-module with $μ(M)=2$ or $3$ then we prove that depth $G(M)\geq d-μ(M)+1$. Our paper is the first systematic study of depth of associated graded modules of MCM modules over hypersurface rings.

math.AC↗
arXiv2022

Graded components of local cohomology modules supported on $\mathfrak{C}$-monomial ideals

Let $A$ be a Dedekind domain of characteristic zero such that its localization at every maximal ideal has mixed characteristic with finite residue field. Let $R=A[X_1,\ldots, X_n]$ be a polynomial ring and $I=(a_1U_1, \ldots, a_c U_c)\subseteq R$ an ideal, where $a_j \in A$ (not necessarily units) and $U_j$'s are monomials in $X_1, \ldots, X_n$. We call such an ideal $I$ as a $\mathfrak{C}$-monomial ideal. Consider the standard multigrading on $R$. We produce a structure theorem for the multigraded components of the local cohomology modules $H^i_I(R)$ for $i \geq 0$. We further analyze the torsion part and the torsion-free part of these components. We show that if $A$ is a PID then each component can be written as a direct sum of its torsion part and torsion-free part. As a consequence, we obtain that their Bass numbers are finite.

math.AC↗
arXiv2022

On the Stable category of maximal Cohen-Macaulay modules over Gorenstein rings

Let $(A,\mathfrak{m})$ be a Gorenstein local ring and let $CMS(A)$ be its stable category of maximal CM $A$-modules. Suppose $CMS(A) \cong CMS(B)$ as triangulated categories. Then we show (1) If $A$ is a complete intersection of codimension $c$ then so is $B$. (2) If $A, B$ are Henselian and not hypersurfaces then $\dim A = \dim B$. (3) If $A, B$ are Henselian and $A$ is an isolated singularity then so is $B$. We also give some applications of our results. It should be remarked that if $R,S$ are complete CM but not necessarily Gorenstein and if there is an triangle isomorphism between the singularity categories of $R$ and $S$ then it is possible that $\dim R - \dim S$ is odd, see M.~Kalck; Adv. Math. 390 (2021), Paper No. 107913.

math.AC↗
arXiv2022

On associated graded modules of maximal Cohen-Macaulay modules over hypersurface rings

Let $(A,\mathfrak{m})$ be a hypersurface ring with dimension $d$, and $M$ a MCM $A-$module with red$(M)\leq 2$ and $μ(M)=2$ or $3$ then we have proved that depth $G(M)\geq d-μ(M)+1$. If $e(A)=3$ and $μ(M)=4$ then in this case we have proved that depth$G(M)\geq d-3$. Next we consider the case when $e(M)=μ(M)i(M)+1$ and prove that depth $G(M)\geq d-1$. When $A = Q/(f)$ where $Q = k[[X_1,\cdots, X_{d+1}]]$ then we give estimates for $\depth G(M)$ in terms of a minimal presentation of $M$. Our paper is the first systematic study of depth of associated graded modules of MCM modules over hypersurface rings.

math.AC↗
arXiv2021

Graded components of Local cohomology modules II

Let $A$ be a commutative Noetherian ring containing a field of characteristic zero. Let $R= A[X_1, \ldots, X_m]$ be a polynomial ring and $A_m(A) = A \langle X_1, \ldots, X_m, \partial_1, \ldots, \partial_m \rangle$ be the $m^{th}$ Weyl algebra over $A$, where $\partial_i=\partial/\partial X_i$. Consider both $R$ and $A_m(A)$ as standard graded with $\mathrm{deg}~ z=0$ for all $z \in A$, $\mathrm{deg}~ X_i=1$, and $\mathrm{deg}~ \partial_i =-1$ for $i=1, \ldots, m$. We present a few results about the behavior of the graded components of local cohomology modules $H_I^i(R)$, where $I$ is an arbitrary homogeneous ideal in $R$. We mostly restrict our attention to the Vanishing, Tameness, and Rigidity properties. To obtain this, we use the theory of $D$-modules and show that generalized Eulerian $A_m(A)$-modules exhibit these properties. As a corollary, we further get that components of graded local cohomology modules with respect to a pair of ideals display similar behavior.

math.AC↗
arXiv2021

Support Varieties and cohomology of Verdier quotients of stable category of complete intersection rings

Let $(A,\mathfrak{m})$ be a complete intersection with $k = A/\mathfrak{m}$ algebraically closed. Let CMS(A) be the stable category of maximal CM $A$-modules. For a large class of thick subcategories $\mathcal{S}$ of CMS(A) we show that there is a theory of support varieties for the Verdier quotient $\mathcal{T} = $ CMS(A)$/\mathcal{S}$. As an application we show that the analogous version of Auslander-Reiten conjecture, Murthys result, Avramov-Buchweitz result on symmetry of vanishing of cohomology holds for $\mathcal{T}$.

math.AC↗
arXiv2021

Complete intersection Approximation, Dual Filtrations and Applications

We give a two step method to study certain questions regarding associated graded module of a Cohen-Macaulay (CM) module $M$ w.r.t an $\mathfrak{m}$-primary ideal $\mathfrak{a}$ in a complete Noetherian local ring $(A,\mathfrak{m})$. The first step, we call it complete intersection approximation, enables us to reduce to the case when both $A$, $ G_\mathfrak{a}(A) = \bigoplus_{n \geq 0} \mathfrak{a}^n/\mathfrak{a}^{n+1} $ are complete intersections and $M$ is a maximal CM $A$-module. The second step consists of analyzing the classical filtration $\{Hom_A(M,\mathfrak{a}^n) \}_{\mathbb{Z}}$ of the dual $Hom_A(M,A)$. We give many applications of this point of view. For instance let $(A,\mathfrak{m})$ be equicharacteristic and CM. Let $a(G_\mathfrak{a}(A))$ be the $a$-invariant of $G_\mathfrak{a}(A)$. We prove: 1. $a(G_\mathfrak{a}(A)) = -\dim A$ iff $\mathfrak{a}$ is generated by a regular sequence. 2. If $\mathfrak{a}$ is integrally closed and $a(G_\mathfrak{a}(A)) = -\dim A + 1$ then $\mathfrak{a}$ has minimal multiplicity. We extend to modules a result of Ooishi relating symmetry of $h$-vectors. As another application we prove a conjecture of Itoh, if $A$ is a CM local ring and $\mathfrak{a}$ is a normal ideal with $e_3^\mathfrak{a}(A) = 0$ then $G_\mathfrak{a}(A)$ is CM.

math.AC↗
arXiv2020

Cohen-Macaulay local rings with $e_2 = e_1-e+1$

In this paper we study Cohen-Macaulay local rings of dimension $d$, multiplicity $e$ and second Hilbert coefficient $e_2$ in the case $e_2 = e_1 - e + 1$. Let $h = μ(\mathfrak{m}) - d$. If $e_2 \neq 0$ then in our case we can prove that type $A \geq e - h -1$. If type $A = e - h -1$ then we show that the associated graded ring $G(A)$ is Cohen-Macaulay. In the next case when type $A = e - h$ we determine all possible Hilbert series of $A$. In this case we show that the Hilbert Series of $A$ completely determines depth $G(A)$.

math.AC↗