Matrix representations for toric parametrizations

In this paper we show that a surface in P^3 parametrized over a 2-dimensional toric variety T can be represented by a matrix of linear syzygies if the base points are finite in number and form locally a complete intersection. This constitutes a direct generalization of the corresponding result over P^2 established in [BJ03] and [BC05]. Exploiting the sparse structure of the parametrization, we obtain significantly smaller matrices than in the homogeneous case and the method becomes applicable to parametrizations for which it previously failed. We also treat the important case T = P^1 x P^1 in detail and give numerous examples.Comment: 20 page

Implicitization of Bihomogeneous Parametrizations of Algebraic Surfaces via Linear Syzygies

We show that the implicit equation of a surface in 3-dimensional projective space parametrized by bi-homogeneous polynomials of bi-degree (d,d), for a given positive integer d, can be represented and computed from the linear syzygies of its parametrization if the base points are isolated and form locally a complete intersection

Resultant of an equivariant polynomial system with respect to the symmetric group

Given a system of n homogeneous polynomials in n variables which is equivariant with respect to the canonical actions of the symmetric group of n symbols on the variables and on the polynomials, it is proved that its resultant can be decomposed into a product of several smaller resultants that are given in terms of some divided differences. As an application, we obtain a decomposition formula for the discriminant of a multivariate homogeneous symmetric polynomial

On the irreducibility of multivariate subresultants

Let $P_1,...,P_n$ be generic homogeneous polynomials in $n$ variables of degrees $d_1,...,d_n$ respectively. We prove that if $\nu$ is an integer satisfying ${\sum_{i=1}^n d_i}-n+1-\min\{d_i\}<\nu,$ then all multivariate subresultants associated to the family $P_1,...,P_n$ in degree $\nu$ are irreducible. We show that the lower bound is sharp. As a byproduct, we get a formula for computing the residual resultant of $\binom{\rho-\nu +n-1}{n-1}$ smooth isolated points in \PP^{n-1}.Comment: Updated version, 4 pages, to appear in CRA

Extraction of cylinders and cones from minimal point sets

We propose new algebraic methods for extracting cylinders and cones from minimal point sets, including oriented points. More precisely, we are interested in computing efficiently cylinders through a set of three points, one of them being oriented, or through a set of five simple points. We are also interested in computing efficiently cones through a set of two oriented points, through a set of four points, one of them being oriented, or through a set of six points. For these different interpolation problems, we give optimal bounds on the number of solutions. Moreover, we describe algebraic methods targeted to solve these problems efficiently

Elimination and nonlinear equations of Rees algebra

A new approach is established to computing the image of a rational map, whereby the use of approximation complexes is complemented with a detailed analysis of the torsion of the symmetric algebra in certain degrees. In the case the map is everywhere defined this analysis provides free resolutions of graded parts of the Rees algebra of the base ideal in degrees where it does not coincide with the corresponding symmetric algebra. A surprising fact is that the torsion in those degrees only contributes to the first free module in the resolution of the symmetric algebra modulo torsion. An additional point is that this contribution -- which of course corresponds to non linear equations of the Rees algebra -- can be described in these degrees in terms of non Koszul syzygies via certain upgrading maps in the vein of the ones introduced earlier by J. Herzog, the third named author and W. Vasconcelos. As a measure of the reach of this torsion analysis we could say that, in the case of a general everywhere defined map, half of the degrees where the torsion does not vanish are understood

An Interview with Hendrik Ehlers of MgM

Hendrik Ehlers discusses the challenges facing demining in Africa, research and development, and mechanical clearance used by his company. His candid replies offer insight into the world of demining and managing a multifaceted organization

THE COMPACT 230 MINECAT, J. Barry Middlesmass Lockwood Beck LTD.

J. Barry Middlemass, Managing Director of Lockwood Beck Limited, has considerable experience in the field of mechanical mine clearance and mine clearance equipment. Before embarking on a career in mine clearance, He served in the military, including reserves, for a total of twenty-nine years, specializing in mines, explosives and improvised explosive devices. When he resigned his commission, he devoted himself fulltime to mechanical demining. As a director of Aardvark for ten years, he had a key role and made a significant contribution to the company\u27s success. Currently, JBM runs his own consulting company, Lockwood Beck, advising a variety of clients on mechanical mine equipment. One of the projects he has been recently involved with is the development of the COMPACT 230 MINECAT

Résultant univarié et courbes algébriques planes

MasterLe premier chapitre traite du résultant de Sylvester qui constitue l'outil essentiel de ce cours. Le deuxième chapitre propose une étude effective du problème de l'intersection de deux courbes algébriques planes: théorème de Bézout, notion de multiplicité d'intersection et calcul de points d'intersection par valeurs et vecteurs propres. Le troisième chapitre aborde la manipulation des courbes algébriques planes rationnelles: degré d'une paramétrisation, problèmes d'implicitation et d'inversion d'une paramétrisation. Enfin, quelques compléments sont donnés sous forme d'exercices dans le dernier chapitre