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Indranath Sengupta

Publications and source records attributed to Indranath Sengupta.

At least 19 recordsLinked to original sources
arXiv2026

Counting Rational Points on Danielewski and Double Danielewski Surfaces over Finite Fields

Let $\Fq$ be the finite field with $q$ elements. We study the number of $\Fq$-rational points on Danielewski and double Danielewski surfaces. For Danielewski surfaces, the point count is reduced to the number of roots of $P(Z)$ over $\Fq.$ For double Danielewski surfaces, one has to count the number of tuples $(\be,\g)\in\Fq^2$, such that $P(0,\g)=0$, $Q(0,\be,\g)=0$ hold simultaneously. We compute these numbers using gcd methods, resultants, character sums, Gauss sums, and the K\"onig--Rados theorem. We obtain explicit formulas in several structured cases, derive general bounds, and give a Macaulay2 algorithm for verification and show an intresting connection between the number of $\Fq$-rational points of these surfaces and polygonal numbers.

math.NT↗
arXiv2025

Tangent Cones of Concatenated Numerical Semigroups

We study the tangent cone at the origin and the Hilbert series for a family of numerical semigroups generated by concatenation of arithmetic sequences. We prove that all the concatenation classes have Cohen-Macaulay tangent cones except the symmetric class, however, the symmetric class does satisfy Rossi's conjecture.

math.AC↗
arXiv2024

Projective Closure of Semigroup Algebras

This paper investigates the projective closure of simplicial affine semigroups in $\mathbb{N}^{d}$, $d \geq 2$. We present a characterization of the Cohen-Macaulay property for the projective closure of these semigroups using Gr\"{o}bner bases. Additionally, we establish a criterion, based on Gr\"{o}bner bases, for determining the Buchsbaum property of non-Cohen-Macaulay projective closures of numerical semigroup rings. Lastly, we introduce the concept of $k$-lifting for simplicial affine semigroups in $\mathbb{N}^d$, and investigate its relationship with the original simplicial affine semigroup.

math.AC↗
arXiv2023

Join of affine semigroups

In this paper, we study the class of affine semigroup generated by integral vectors, whose components are in generalised arithmetic progression and we observe that the defining ideal is determinantal. We also give a sufficient condition on the defining ideal of the semigroup ring for the equality of the Betti numbers of the defining ideal and those of its initial ideal. We introduce the notion of an affine semigroup generated by join of two affine semigroups and show that this affine semigroup exhibits some nice properties including Cohen-Macaulayness.

math.AC↗
arXiv2023

Affine semigroups of maximal projective dimension-II

If the Krull dimension of the semigroup ring is greater than one, then affine semigroups of maximal projective dimension ($\mathrm{MPD}$) are not Cohen-Macaulay, but they may be Buchsbaum. We give a necessary and sufficient condition for simplicial $\mathrm{MPD}$-semigroups to be Buchsbaum in terms of pseudo-Frobenius elements. We give certain characterizations of $\prec$-almost symmetric $\mathcal{C}$-semigroups. When the cone is full, we prove the irreducible $\mathcal{C}$-semigroups, and $\prec$-almost symmetric $\mathcal{C}$-semigroups with Betti-type three satisfy the extended Wilf's conjecture. For $e \geq 4$, we give a class of MPD-semigroups in $\mathbb{N}^2$ such that there is no upper bound on the Betti-type in terms of embedding dimension $e$. Thus, the Betti-type may not be a bounded function of the embedding dimension. We further explore the submonoids of $\mathbb{N}^d$, which satisfy the Arf property.

math.AC↗
arXiv2023

The $\mathrm{v}$-Number of Binomial Edge Ideals

The invariant $\mathrm{v}$-number was introduced very recently in the study of Reed-Muller-type codes. Jaramillo and Villarreal (J Combin. Theory Ser. A 177:105310, 2021) initiated the study of the $\mathrm{v}$-number of edge ideals. Inspired by their work, we take the initiation to study the $\mathrm{v}$-number of binomial edge ideals in this paper. We discuss some properties and bounds of the $\mathrm{v}$-number of binomial edge ideals. We explicitly find the $\mathrm{v}$-number of binomial edge ideals locally at the associated prime corresponding to the cutset $\emptyset$. We show that the $\mathrm{v}$-number of Knutson binomial edge ideals is less than or equal to the $\mathrm{v}$-number of their initial ideals. Also, we classify all binomial edge ideals whose $\mathrm{v}$-number is $1$. Moreover, we try to relate the $\mathrm{v}$-number with the Castelnuvo-Mumford regularity of binomial edge ideals and give a conjecture in this direction.

math.AC↗
arXiv2022

Projective closures of affine monomial curves

We study the projective closures of three important families of affine monomial curves in dimension $4$, namely the Backelin curve, the Bresinsky curve and the Arslan curve, in order to explore possible connections between syzygies and the arithmetic Cohen-Macaulay property.

math.AC↗
arXiv2022

Cohen-Macaulay Property of Binomial Edge Ideals with Girth of Graphs

Conca and Varbaro (Invent. Math. 221 (2020), no. 3) showed the equality of depth of a graded ideal and its initial ideal in a polynomial ring when the initial ideal is square-free. In this paper, we give some beautiful applications of this fact in the study of Cohen-Macaulay binomial edge ideals. We prove that for the characterization of Cohen-Macaulay binomial edge ideals, it is enough to consider only "biconnected graphs with some whisker attached" and this done by investigating the initial ideals. We give several necessary conditions for a binomial edge ideal to be Cohen-Macaulay in terms of smaller graphs. Also, under a hypothesis, we give a sufficient condition for Cohen-Macaulayness of binomial edge ideals in terms of blocks of graphs. Moreover, we show that a graph with Cohen-Macaulay binomial edge ideal has girth less than $5$ or equal to infinity.

math.AC↗
arXiv2022

Derivation module and the Hilbert-Kunz multiplicity of the co-ordinate ring of a projective monomial curve

Let $n_0, n_1, \ldots, n_p$ be a sequence of positive integers such that $n_0 < n_1 < \cdots < n_p$ and $\mathrm{gcd}(n_0,n_1, \ldots,n_p) = 1$. Let $S = \langle (0,n_p), (n_0,n_p-n_0),\ldots,(n_{p-1},n_p-n_{p-1}), (n_p,0) \rangle$ be an affine semigroup in $\mathbb{N}^2$. The semigroup ring $k[S]$ is the co-ordinate ring of the projective monomial curve in the projective space $\mathbb{P}_k^{p+1}$, which is defined parametrically by \begin{center} $x_0 = v^{n_p}, \quad x_1 = u^{n_0}v^{n_p-n_0},\quad \ldots , \quad x_p= u^{n_{p-1}}v^{n_p-n_{p-1}}, \quad x_{p+1} = u^{n_p}$. \end{center} In this article, we consider the case when $n_0, n_1, \ldots, n_p$ forms an arithmetic sequence, and give an explicit set of minimal generators for the derivation module $\mathrm{Der}_k(k[S])$. Further, we give an explicit formula for the Hilbert-Kunz multiplicity of the co-ordinate ring of a projective monomial curve.

math.AC↗
arXiv2022

On the Associated Graded ring of Semigroup Algebras

In this paper, we give the necessary and sufficient conditions for the Cohen-Macaulayness of the associated graded ring of a simplicial affine semigroups using Gröbner basis. We generalize the concept of homogeneous numerical semigroup for the simplicial affine semigroup and show that the Betti numbers of the corresponding semigroup ring matches with the Betti numbers of the associated graded ring. We also define the nice extension for simplicial affine semigroups, motivated by the notion of a nice extension of the numerical semigroups.

math.AC↗
arXiv2022

Closed Cohen-Macaulay completion of binomial edge ideals

Let $\mathbf{CCM}$ denote the class of closed graphs with Cohen-Macaulay binomial edge ideals and $\mathbf{PIG}$ denote the class of proper interval graphs. Then $\mathbf{CCM}\subseteq \mathbf{PIG}$. The $\mathbf{PIG}$-completion problem is a classical problem in molecular biology as well as in graph theory and this problem is known to be NP-hard. In this paper, we study the $\mathbf{CCM}$-completion problem. We give a method to construct all possible $\mathbf{CCM}$-completion of a graph. We find the $\mathbf{CCM}$-completion number and the set of all minimal $\mathbf{CCM}$-completions for a large class of graphs. Moreover, for that class, we give a polynomial-time algorithm to compute the $\mathbf{CCM}$-completion number and a minimum $\mathbf{CCM}$-completion of a given graph. We investigate unmixed and Cohen-Macaulay properties of binomial edge ideals of induced subgraphs. Also, we discuss the accessible graphs completion and the Cohen-Macaulay property of binomial edge ideals of whisker graphs.

math.AC↗
arXiv2022

On Row-Factorization relations of certain numerical semigroups

Let $H$ be a numerical semigroup minimally generated by an almost arithmetic sequence. We give a description of a possible row-factorization $(\RF)$ matrix for each pseudo-Frobenius element of $H.$ Further, when $H$ is symmetric and has embedding dimension 4 or 5, we prove that the defining ideal is minimally generated by $\RF$-relations.

math.AC↗
arXiv2022

Affine semigroups of maximal projective dimension

We generalize the notion of symmetric semigroups, pseudo symmetric semigroups, and row factorization matrices for pseudo Frobenius elements of numerical semigroups to the case of semigroups with maximal projective dimension (MPD semigroups).

math.AC↗
arXiv2022

On the algebraic invariants of certain affine semigroup algebras

Let $a$ and $d$ be two linearly independent vectors in $\mathbb{N}^2$, over the field of rational numbers. For a positive integer $k \geq 2$, consider the sequence $a, a+d, \ldots, a+kd$ such that the affine semigroup $S_{a,d,k} = \langle a, a+d, \ldots, a+kd \rangle$ is minimally generated by this sequence. We study the properties of affine semigroup algebra $k[S_{a,d,k}]$ associated to this semigroup. We prove that $k[S_{a,d,k}]$ is always Cohen-Macaulay and it is Gorenstein if and only if $k=2$. For $k=2,3,4$, we explicitly compute the syzygies, minimal graded free resolution and Hilbert series of $k[S_{a,d,k}].$ We also give a minimal generating set and a Gröbner basis of the defining ideal of $k[S_{a,d,k}].$ Consequently, we prove that $k[S_{a,d,k}]$ is Koszul. Finally, we prove that the Castelnuovo-Mumford regularity of $k[S_{a,d,k}]$ is $1$ for any $a,d,k.$

math.AC↗
arXiv2022

Projective Closure of Affine Monomial Curves II

In this paper our aim is twofold. First, we introduce the notion of star gluing of numerical semigroups and show that arithmetically Cohen-Macaulay and Gorenstein properties of the projective closure are preserved under this gluing operation. We then give a condition on Gröbner basis of the defining ideal of an affine monomial curve which ensures that the Betti sequence of the affine curve is the same as the Betti sequence of its projective closure. We also study the effect of simple gluing on Betti sequences of the projective closure. Finally, we construct some numerical semigroups, using a gluing technique, such that the Cohen-Macaulay type of corresponding affine curve and its projective closure are both $n$.

math.AC↗
arXiv2022

Cohen-Macaulay Binomial edge ideals in terms of blocks with whiskers

For a graph $G$, Bolognini et al. have shown $J_{G}$ is strongly unmixed $\Rightarrow$ $J_{G}$ is Cohen-Macaulay $\Rightarrow$ $G$ is accessible, where $J_{G}$ denotes the binomial edge ideals of $G$. Accessible and strongly unmixed properties are purely combinatorial. We give some motivations to focus only on blocks with whiskers for the characterization of all $G$ with Cohen-Macaulay $J_{G}$. We show that accessible and strongly unmixed properties of $G$ depend only on the corresponding properties of its blocks with whiskers and vice versa. Also, we give an infinite class of graphs whose binomial edge ideals are Cohen-Macaulay, and from that, we classify all $r$-regular $r$-connected graphs such that attaching some special whiskers to it, the binomial edge ideals become Cohen-Macaulay. Finally, we define a new class of graphs, called \textit{strongly $r$-cut-connected} and prove that the binomial edge ideal of any strongly $r$-cut-connected accessible graph having at most three cut vertices is Cohen-Macaulay.

math.AC↗
arXiv2022

The $\mathrm{v}$-number of Monomial Ideals

We generalize some results of $\mathrm{v}$-number for arbitrary monomial ideals by showing that the $\mathrm{v}$-number of an arbitrary monomial ideal is the same as the $\mathrm{v}$-number of its polarization. We prove that the $\mathrm{v}$-number $\mathrm{v}(I(G))$ of the edge ideal $I(G)$, the induced matching number $\mathrm{im}(G)$ and the regularity $\mathrm{reg}(R/I(G))$ of a graph $G$, satisfy $\mathrm{v}(I(G))\leq \mathrm{im}(G)\leq \mathrm{reg}(R/I(G))$, where $G$ is either a bipartite graph, or a $(C_{4},C_{5})$-free vertex decomposable graph, or a whisker graph. There is an open problem in \cite{v}, whether $\mathrm{v}(I)\leq \mathrm{reg}(R/I)+1$ for any square-free monomial ideal $I$. We show that $\mathrm{v}(I(G))>\mathrm{reg}(R/I(G))+1$, for a disconnected graph $G$. We derive some inequalities of $\mathrm{v}$-numbers which may be helpful to answer the above problem for the case of connected graphs. We connect $\mathrm{v}(I(G))$ with an invariant of the line graph $L(G)$ of $G$. For a simple connected graph $G$, we show that $\mathrm{reg}(R/I(G))$ can be arbitrarily larger than $\mathrm{v}(I(G))$. Also, we try to see how the $\mathrm{v}$-number is related to the Cohen-Macaulay property of square-free monomial ideals.

math.AC↗