Juan Gerardo Alcázar

Computation of the topology of real algebraic space curves

An algorithm for computing the topology of a real algebraic space curve C, implicitly defined as the intersection of two surfaces, is presented. Given C, the algorithm generates a space graph which is topologically equivalent to the real variety on the Euclidean space. The algorithm is based on the computation of the graphs of at most two projections of C. For this purpose, we introduce the notion of space general position for space curves, we show that any curve under the above conditions can always be linearly transformed to be in general position, and we present effective methods for checking whether space general position has been reached.

A delineability-based method for computing critical sets of algebraic surfaces

In this paper, we address the problem of determining a real finite set of z-values where the topology type of the level curves of a (maybe singular) algebraic surface may change. We use as a fundamental and crucial tool McCallums theorem on analytic delineability of polynomials (see [McCallum, S., 1998. An improved projection operation for cylindrical algebraic decomposition. In: Caviness, B.F., Johnson, J.R. (Eds.), Quantifier Elimination and Cylindrical Algebraic Decomposition. Springer Verlag, pp. 242-268]). Our results allow to algorithmically compute this finite set by analyzing the real roots of a univariate polynomial; namely, the double discriminant of the implicit equation of the surface. As a consequence, an application to offsets is shown.

Detecting symmetries of rational plane and space curves

Abstract This paper addresses the problem of determining the symmetries of a plane or space curve defined by a rational parametrization. We provide effective methods to compute the involution and rotation symmetries for the planar case. As for space curves, our method finds the involutions in all cases, and all the rotation symmetries in the particular case of Pythagorean-hodograph curves. Our algorithms solve these problems without converting to implicit form. Instead, we make use of a relationship between two proper parametrizations of the same curve, which leads to algorithms that involve only univariate polynomials. These algorithms have been implemented and tested in the Sage system.

Local shape of offsets to algebraic curves

In this paper we introduce the notion of local shape to describe the behavior of a real place of an algebraic curve around its center. We analyze how the local shape is affected by the offsetting process, and we relate this phenomenon to the curvature of the curve. Furthermore, we characterize the situations when the offsetting process behaves locally well, so that the local shape is preserved.

Detecting similarity of rational plane curves

A novel and deterministic algorithm is presented to detect whether two given rational plane curves are related by means of a similarity, which is a central question in Pattern Recognition. As a by-product it finds all such similarities, and the particular case of equal curves yields all symmetries. A complete theoretical description of the method is provided, and the method has been implemented and tested in the Sage system for curves of moderate degrees.

Efficient detection of symmetries of polynomially parametrized curves

We present efficient algorithms for detecting central and mirror symmetry for the case of algebraic curves defined by means of polynomial parametrizations. The algorithms are based on the existence of a linear relationship between two proper polynomial parametrizations of the curve, which leads to a triangular polynomial system (with complex unknowns) that can be solved in a very fast way; in particular, curves parametrized by polynomials of serious degrees can be analyzed in a few seconds. In our analysis we provide a good number of theoretical results on symmetries of polynomial curves, algorithms for detecting rotation and mirror symmetry, and closed formulas to determine the symmetry center and the symmetry axis, when they exist. A complexity analysis of the algorithms is also given.

Topology of 2D and 3D rational curves

In this paper we present algorithms for computing the topology of planar and space rational curves defined by a parametrization. The algorithms given here work directly with the parametrization of the curve, and do not require to compute or use the implicit equation of the curve (in the case of planar curves) or of any projection (in the case of space curves). Moreover, these algorithms have been implemented in Maple; the examples considered and the timings obtained show good performance skills.

Computing the shapes arising in a family of space rational curves depending on one parameter

In a previous work (Alcazar, 2009) we addressed the problem of determining the topology types arising in a family of plane rational curves depending on one parameter, defined by means of a rational parametrization. In this paper, starting from the ideas in Alcazar (2009) we address the analogous problem for families of space rational curves, also defined by means of a rational parametrization. Hence, the main result in the paper is an algorithm for computing the (finitely many) real values of the parameter where the topology of the family may change. The algorithm has been implemented in the computer algebra system Maple 13, with good practical results; several examples and timings are provided.

Symmetry detection of rational space curves from their curvature and torsion

We present a novel, deterministic, and efficient method to detect whether a given rational space curve is symmetric. By using well-known differential invariants of space curves, namely the curvature and torsion, the method is significantly faster, simpler, and more general than an earlier method addressing a similar problem (Alcazar et al., 2014b). To support this claim, we present an analysis of the arithmetic complexity of the algorithm and timings from an implementation in Sage. The paper presents a novel, deterministic, and efficient method to detect whether a given rational space curve is symmetric.The method is significantly faster, simpler, and more general than earlier methods addressing similar problems.An analysis of the arithmetic complexity of the algorithm and timings from an implementation in Sage are included.

On the different shapes arising in a family of plane rational curves depending on a parameter

Given a family of plane rational curves depending on a real parameter, defined by its parametric equations, we provide an algorithm to compute a finite partition of the parameter space (R, in general) so that the shape of the family stays invariant along each element of the partition. So, from this partition the topology types in the family can be determined. The algorithm is based on a geometric interpretation of previous work (Alcazar et al., 2007) for the implicit case. However, in our case the algorithm works directly with the parametrization of the family, and the implicit equation does not need to be computed. Timings comparing the algorithm in the implicit and the parametric cases are given; these timings show that the parametric algorithm developed here provides in general better results than the known algorithm for the implicit case.