# Ignacio OjedaUniversidad de Extremadura | UNEX · Department of Mathematics

Ignacio Ojeda

PhD in Mathematics

## About

57

Publications

4,592

Reads

**How we measure 'reads'**

A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more

322

Citations

Introduction

Additional affiliations

October 2000 - present

## Publications

Publications (57)

We recall and delve into the different characterizations of the depth of an affine semigroup ring, providing an original characterization of depth two in three and four dimensional cases which are closely related to the existence of a maximal element in certain Apery sets.

For family $x'=(a_0+a_1\cos t+a_2 \sin t)|x|+b_0+b_1 \cos t+b_2 \sin t$, we solve three basic problems related with its dynamics. First, we characterize when it has a center (Poincar\'e center focus problem). Second, we show that each equation has a finite number of limit cycles (finiteness problem), and finally we give a uniform upper bound for th...

A numerical semigroup is said to be universally free if it is free for any possible arrangement of its minimal generating set. In this work, we establish that toric ideals associated with universally free numerical semigroups can be generated by their set of circuits. Additionally, we provide a characterization of universally free numerical semigro...

A criterion is obtained for the semi-stability of the isolated singular positive closed solutions, i.e., singular positive limit cycles, of the Abel equation $x'=A(t)x^3+B(t)x^2$, where $A,B$ are smooth functions with two zeros in the interval $[0,T]$ and where these singular positive limit cycles satisfy certain conditions, which allows an upper b...

We study the number of rational limit cycles of the Abel equation $x'=A(t)x^3+B(t)x^2$, where $A(t)$ and $B(t)$ are real trigonometric polynomials. We show that this number is at most the degree of $A(t)$ plus one.

In this paper, we introduce and study the numerical semigroups generated by $$\{a_1, a_2, \ldots \} \subset {\mathbb {N}}$$ { a 1 , a 2 , … } ⊂ N such that $$a_1$$ a 1 is the repunit number in base $$b > 1$$ b > 1 of length $$n > 1$$ n > 1 and $$a_i - a_{i-1} = a\, b^{i-2},$$ a i - a i - 1 = a b i - 2 , for every $$i \ge 2$$ i ≥ 2 , where a is a po...

We study the rational solutions of the Abel equation x′=A(t)x3+B(t)x2 where A and B∈C[t]. We prove that if deg(A) is even or deg(B)>(deg(A)−1)/2 then the equation has at most two rational solutions. For any other case, an upper bound on the number of rational solutions is obtained. Moreover, we prove that if there are more than (deg(A)+1)/2 rat...

Let a,b and n>1 be three positive integers such that a and ∑j=0n−1bj are relatively prime. In this paper, we prove that the toric ideal I associated to the submonoid of N generated by {∑j=0n−1bj}∪{∑j=0n−1bj+a∑j=0i−2bj∣i=2,…,n} is determinantal. Moreover, we prove that for n>3, the ideal I has a unique minimal system of generators if and only if a<b...

In this paper, we introduce and study the numerical semigroups generated by $\{a_1, a_2, \ldots \} \subset \mathbb{N}$ such that $a_1$ is the repunit number in base $b > 1$ of length $n > 1$ and $a_i - a_{i-1} = a\, b^{i-2},$ for every $i \geq 2$, where $a$ is a positive integer relatively prime with $a_1$. These numerical semigroups generalize the...

Let $a, b$ and $n > 1$ be three positive integers such that $a$ and $\sum_{j=0}^{n-1} b^j$ are relatively prime. In this paper, we prove that the toric ideal $I$ associated to the submonoid of $\mathbb{N}$ generated by $\{\sum_{j=0}^{n-1} b^j\} \cup \{\sum_{j=0}^{n-1} b^j + a\, \sum_{j=0}^{i-2} b^j \mid i = 2, \ldots, n\}$ is determinantal. Moreove...

We study the rational solutions of the Abel equation $x'=A(t)x^3+B(t)x^2$ where $A,B\in C[t]$. We prove that if $deg(A)$ is even or $deg(B)>(deg(A)-1)/2$ then the equation has at most two rational solutions. For any other case, an upper bound on the number of rational solutions is obtained. Moreover, we prove that if there are more than $(deg(A)+1)...

We propose necessary and sufficient conditions for an integer matrix to be decomposable in terms of its Hermite normal form. Specifically, to each integer matrix, we associate a symmetric integer matrix whose reducibility can be efficiently determined by elementary linear algebra techniques, and which completely determines the decomposability of th...

In this paper, we study a family of binomial ideals defining monomial curves in the n -dimensional affine space determined by n hypersurfaces of the form $x_i^{c_i} - x_1^{u_{i1}} \cdots x_n^{u_{1n}}$ in $\Bbbk [x_1, \ldots , x_n]$ with $u_{ii} = 0, \ i\in \{ 1, \ldots , n\}$ . We prove that the monomial curves in that family are set-theoretic comp...

In this paper we introduce the notion of extension of a numerical semigroup. We provide a characterization of the numerical semigroups whose extensions are all arithmetic and we give an algorithm for the computation of the whole set of arithmetic extension of a given numerical semigroup. As by-product, new explicit formulas for the Frobenius number...

In this paper we introduce the notion of extension of a numerical semigroup. We provide a characterization of the numerical semigroups whose extensions are all arithmetic and we give an algorithm for the computation of the whole set of arithmetic extension of a given numerical semigroup. As by-product, new explicit formulas for the Frobenius number...

A \emph{congruence} on $\mathbb{N}^n$ is an equivalence relation on $\mathbb{N}^n$ that is compatible with the additive structure. If $\Bbbk$ is a field, and $I$ is a \emph{binomial ideal} in $\Bbbk[X_1,\dots,X_n]$ (that is, an ideal generated by polynomials with at most two terms), then $I$ induces a congruence on $\mathbb{N}^n$ by declaring $\mat...

We propose necessary and sufficient conditions for an integer matrix to be decomposable in terms of its Hermite normal form. Specifically, to each integer matrix of maximal row rank without columns of zeros, we associate a symmetric whole matrix whose reducibility can be determined by elementary Linear Algebra, and which completely determines the d...

In this expository note, we give a self-contained presentation of the equivalence between the opposite category of commutative monoids and that of commutative, monoid \(\Bbbk \)-schemes that are diagonalizable, for any field \(\Bbbk \).

In this paper we study those submonoids of \(\mathbb {N}^d\) with a nontrivial pseudo-Frobenius set. In the affine case, we prove that they are the affine semigroups whose associated algebra over a field has maximal projective dimension possible. We prove that these semigroups are a natural generalization of numerical semigroups and, consequently,...

A simple way of computing the Ap\'ery set of a numerical semigroup (or monoid) with respect to a generator, using Groebner bases, is presented, together with a generalization for affine semigroups. This computation allows us to calculate the type set and, henceforth, to check the Gorenstein condition which characterizes the symmetric numerical subg...

We study the structure of the family of numerical semigroups with fixed multiplicity and Frobenius number. We give an algorithmic method to compute all the semigroups in this family. As an application we compute the set of all numerical semigroups with given multiplicity and genus.

We establish a one-to-one correspondence between numerical semigroups of genus $g$ and almost symmetric numerical semigroups with Frobenius number $F$ and type $F-2g$, provided that $F$ is greater than $4g-1$.

In this paper we study those submonoids of $\mathbb{N}^d$ which a non-trivial pseudo-Frobenius set. In the affine case, we prove that they are the affine semigroups whose associated algebra over a field has maximal projective dimension possible. We prove that these semigroups are a natural generalization of numerical semigroups and, consequently, m...

We give two algorithmic procedures to compute the whole set of almost symmetric numerical semigroups with fixed Frobenius number and type, and the whole set of almost symmetric numerical semigroups with fixed Frobenius number. Our algorithms allow to compute the whole set of almost symmetric numerical semigroups with fixed Frobenius number with sim...

A congruence on \(\mathbb {N}^n\) is an equivalence relation on \(\mathbb {N}^n\) that is compatible with the additive structure. If \(\Bbbk \) is a field, and I is a binomial ideal in \(\Bbbk [X_1,\dots ,X_n]\) (that is, an ideal generated by polynomials with at most two terms), then I induces a congruence on \(\mathbb {N}^n\) by declaring u and v...

We give two algorithmic procedures to compute the whole set of almost symmetric numerical semigroups with fixed Frobenius number and type, and the whole set of almost symmetric numerical semigroups with fixed Frobenius number. Our algorithms allow to compute the whole set of almost symmetric numerical semigroups with fixed Frobenius number with sim...

This article deals with the study of the number of limit cycles surrounding a critical point of a quadratic planar vector field, which, in normal form, can be written as $x'= a_1 x-y-a_3x^2+(2 a_2+a_5)xy + a_6 y^2$, $y'= x+a_1 y + a_2x^2+(2 a_3+a_4)xy -a_2y^2$. In particular, we study the semi-varieties defined in terms of the parameters $a_1,a_2,\...

In this paper, we study a family of binomial ideals defining monomial curves in the $n-$dimensional affine space determined by $n$ hypersurfaces of the form $x_i^{c_i} - x_1^{u_{i1}} \cdots x_n^{u_{1n}} \in k[x_1, \ldots, x_n]$ with $u_{ii} = 0$, $i\in \{ 1, \ldots, n\}$. We prove that, the monomial curves in that family are set-theoretic complete...

This note describes in a complete way the short resolution given Proc. Amer.
Math. Soc. \textbf{131}, 4, (2003), 1081--1091. In that paper the results are
claimed to be valid for any lattice ideal. However, the main result assumes
implicitly an hypothesis of simpliciality on the semigroup defining the lattice
ideal. Now, we omit this hypothesis and...

Let S1 and S2 be two affine semigroups, and let S be the gluing of S1 and S2. Several invariants of S are related to those of S1 and S2; we review some of the most important properties preserved under gluings. The aim of this paper is to prove that this is the case for the Frobenius vector and the Hilbert series. Applications to complete intersecti...

A simple way of computing the Apéry set of a numerical semigroup (or monoid) with respect to a generator, using Groebner bases, is presented, together with a generalization for affine semigroups. This computation allows us to calculate the type set and, henceforth, to check the Gorenstein condition which characterizes the symmetric numerical subgro...

In this paper a new classification of monomial curves in
$\mathbb{A}^4(\mathbbmss{k})$ is given. Our classification relies on the
detection of those binomials and monomials that have to appear in every system
of binomial generators of the defining ideal of the monomial curve; these
special binomials and monomials are called indispensable in the lit...

Let $S_1$ and $S_2$ be two affine semigroups and let $S$ be the gluing of $S_1$ and $S_2$. Several invariants of $S$ are then related to those of $S_1$ and $S_2$; we review some of the most important properties preserved under gluings. The aim of this paper is to prove that this is the case for the Frobenius vector and the Hilbert series. Applicati...

By introducing a new matrix operator (the block vectorization), I give a
necessary and sufficient condition for factorization of a matrix into the
Kronecker product of two other matrices. As a consequence, I obtain an
elementary algorithmic procedure to decide whether a matrix have square root
for the Kronecker product.

We characterize affine semigroups having one Betti element and we compute
some relevant non-unique factorization invariants for these semigroups. As an
example, we particularize our description to numerical semigroups.

Let $\mathbb{N} \mathcal{A}$ be the monoid generated by $\mathcal{A} =
{\mathbf{a}_1, ..., \mathbf{a}_n} \subseteq \mathbb{Z}^d.$ We introduce the
homogeneous catenary degree of $\mathbb{N} \mathcal{A}$ as the smallest $N \in
\mathbb N$ with the following property: for each $\mathbf{a} \in \mathbb{N}
\mathcal{A}$ and any two factorizations $\mathbf...

In this paper, we prove that every binomial ideal in a polynomial ring over an algebraically closed field of characteristic zero admits a canonical primary decomposition into binomial ideals. Moreover, we prove that this special decomposition is obtained from a cellular decomposition which is also defined in a canonical way and does not depend on t...

A finitely generated commutative monoid is uniquely presented if it has only a minimal presentation. We give necessary and sufficient conditions for finitely generated, combinatorially finite, cancellative, commutative monoids to be uniquely presented. We use the concept of gluing to construct commutative monoids with this property. Finally for som...

In this paper, we deal with the problem of uniqueness of minimal system of binomial generators of a semigroup ideal. Concretely, we give different necessary and/or sufficient conditions for uniqueness of such minimal system of generators. These conditions come from the study and combinatorial description of the so-called indispensable binomials in...

In this paper, we study double structures supported on rational normal curves. After recalling the general construction of double structures supported on a smooth curve described in \cite{fer}, we specialize it to double structures on rational normal curves. To every double structure we associate a triple of integers $ (2r,g,n) $ where $ r $ is the...

This paper is concerned with the combinatorial description of the graded minimal free resolution of certain monomial algebras which includes toric rings. Concretely, we explicitly describe how the graded minimal free resolution of those algebras is related to the combinatorics of some simplicial complexes. Our description may be interpreted as an a...

In Peeva and Sturmfels (199811.
Peeva , I. ,
Sturmfels , B. ( 1998 ). Generic lattice ideals . J. Amer. Math. Soc. 11 : 363 – 373 . View all references), the authors introduce a new notion of genericity for lattice ideals; however, the deterministic construction of this kind of “generic lattice ideals” with prescribed properties (such as Betti nu...

After establishing bounds on the Rao function and on the genus of projective curves that generalize the ones in [5] and in [12], we describe the even G-liaison classes of some unions of curves attaining the bounds, and of more general unions with analogous geometric properties. In particular, we prove that their Hartshorne–Rao module identifies the...

In this paper, we study the Hilbert scheme of non degenerate locally Cohen- Macaulay projective curves with general hyperplane section spanning a linear space of dimension 2 and minimal Hilbert function. The main result is that those curves are almost always the general element of a generically smooth component H
n,d,g
of the corresponding Hilbert...

We characterize the hull resolution of a monomial curve in three-dimensional affine space, and we compare this resolution with the minimal one. Concretely, we give a necessary and sufficient condition for the minimality of the hull resolution of a monomial curve in three-dimensional affine space in terms of the associated semigroup. We also get a l...

In this paper, we study and compute bounds for the index of nilpotency of lattice and cellular binomial ideals which are optimal in many cases. This computations can be generalized to binomial ideals, getting an effective Hilbert's Nullstellensatz for binomial ideals.

Eisenbud and Sturmfels’ theoretical study assures that it is possible to find a primary decomposition of binomial ideals into binomial ideals over an algebraically closed field. In this paper we complete the algorithms in Eisenbud and Sturmfels (1996, Duke Math. J., 84, 1–45) by filling in the steps for which the authors say they have not been very...

V.Ortiz established in (10) the existence of a canoni- cal decomposition of ideals in a commutative noetherian ring. In this paper we study the canonical decomposition of ideals in a polynomial ring and we give an algorithmic procedure to compute canonical decompositions.