Real Algebraic Curves Research Articles

Real Algebraic Curves on Real del Pezzo Surfaces

AbstractThe study of the topology of real algebraic varieties dates back to the work of Harnack, Klein, and Hilbert in the 19th century; in particular, the isotopy-type classification of real algebraic curves in real toric surfaces is a classical subject that has undergone considerable evolution. On the other hand, not much is known for more general ambient surfaces. We take a step forward in the study of topological-type classification of real algebraic curves on non-toric surfaces focusing on real del Pezzo surfaces of degree 1 and 2 with multi-components real part. We use degeneration methods and real enumerative geometry in combination with variations of classical methods to give obstructions to the existence of topological-type classes realized by real algebraic curves and to give constructions of real algebraic curves with prescribed topology.

The separating semigroup of a real curve

We introduce the separating semigroup of a real algebraic curve of dividing type. The elements of this semigroup record the possible degrees of the covering maps obtained by restricting separating morphisms to the real part of the curve. We also introduce the hyperbolic semigroup which consists of elements of the separating semigroup arising from morphisms which are compositions of a linear projection with an embedding of the curve to some projective space. We completely determine both semigroups in the case of maximal curves. We also prove that any embedding of a real curve to projective space of sufficiently high degree is hyperbolic. Using these semigroups we show that the hyperbolicity locus of an embedded curve is in general not connected.

Smooth rational affine varieties with infinitely many real forms

Abstract We construct smooth rational real algebraic varieties of every dimension ≥ 4 {\geq 4} which admit infinitely many pairwise non-isomorphic real forms.

Voronoi cells of varieties

Every real algebraic variety determines a Voronoi decomposition of its ambient Euclidean space. Each Voronoi cell is a convex semialgebraic set in the normal space of the variety at a point. We compute the algebraic boundaries of these Voronoi cells.

Measuring the local non-convexity of real algebraic curves

The goal of this paper is to measure the non-convexity of compact and smooth connected components of real algebraic plane curves. We study these curves first in a general setting and then in an asymptotic one. In particular, we consider sufficiently small levels of a real bivariate polynomial in a small enough neighbourhood of a strict local minimum at the origin of the real affine plane. We introduce and describe a new combinatorial object, called the Poincaré-Reeb graph, whose role is to encode the shape of such curves and allow us to quantify their non-convexity. Moreover, we prove that in this setting the Poincaré-Reeb graph is a plane tree and can be used as a tool to study the asymptotic behaviour of level curves near a strict local minimum. Finally, using the real polar curve, we show that locally the shape of the levels stabilises and that no spiralling phenomena occur near the origin.

Expected number and distribution of critical points of real Lefschetz pencils

Dans cet article, on donne une formule de Riemann–Hurwitz asymptotique et probabiliste qui calcule la valeur attendue de l’indice de ramification réel d’un revêtement aléatoire de la sphère de Riemann. Plus généralement, on étudie l’asymptotique de la valeur attendue du nombre et de la distribution des points critiques réels d’un pinceau de Lefschetz réel sur une variété algébrique réelle. Tout au long de l’article, on donne des résultats analogues pour le cas complexe. Notre outil principal est la théorie des sections pics d’Hörmander.

The polynomial method over varieties

We establish sharp estimates that adapt the polynomial method to arbitrary varieties. These include a partitioning theorem, estimates on polynomials vanishing on fixed sets and bounds for the number of connected components of real algebraic varieties. As a first application, we provide a general incidence estimate that is tight in its dependence on the size, degree and dimension of the varieties involved.

Algebraic models of the line in the real affine plane

We study smooth rational closed embeddings of the real affine line into the real affine plane, that is algebraic rational maps from the real affine line to the real affine plane which induce smooth closed embeddings of the real euclidean line into the real euclidean plane. We consider these up to equivalence under the group of birational automorphisms of the real affine plane which are diffeomorphisms of its real locus. We show that in contrast with the situation in the categories of smooth manifolds with smooth maps and of real algebraic varieties with regular maps where there is only one equivalence class up to isomorphism, there are non-equivalent smooth rational closed embeddings up to such birational diffeomorphisms.

Visualizing Planar and Space Implicit Real Algebraic Curves with Singularities

We present a new method for visualizing implicit real algebraic curves inside a bounding box in the $2$-D or $3$-D ambient space based on numerical continuation and critical point methods. The underlying techniques work also for tracing space curve in higher-dimensional space. Since the topology of a curve near a singular point of it is not numerically stable, we trace only the curve outside neighborhoods of singular points and replace each neighborhood simply by a point, which produces a polygonal approximation that is $\epsilon$-close to the curve. Such an approximation is more stable for defining the numerical connectedness of the complement of the projection of the curve in $\mathbb{R}^2$, which is important for applications such as solving bi-parametric polynomial systems. The algorithm starts by computing three types of key points of the curve, namely the intersection of the curve with small spheres centered at singular points, regular critical points of every connected components of the curve, as well as intersection points of the curve with the given bounding box. It then traces the curve starting with and in the order of the above three types of points. This basic scheme is further enhanced by several optimizations, such as grouping singular points in natural clusters, tracing the curve by a try-and-resume strategy and handling "pseudo singular points". The effectiveness of the algorithm is illustrated by numerous examples. This manuscript extends our preliminary results that appeared in CASC 2018.

Quotients and invariants of $$\mathcal {AS}$$-sets equipped with a finite group action

Using the geometric quotient of a real algebraic set by the action of a finite group G, we construct invariants of G-affine real algebraic varieties with respect to equivariant homeomorphisms with algebraic graph, including additive invariants with values in $${\mathbb {Z}}$$ . The construction requires to consider the wider category of $$\mathcal {AS}$$ -sets.

Products of real equivariant weight filtrations

We first show the existence of a weight filtration on the equivariant cohomology of real algebraic varieties equipped with the action of a finite group, by applying group cohomology to the dual geometric filtration. We then prove the compatibility of the equivariant weight filtrations and spectral sequences with Kunneth isomorphism, cup and cap products, from the filtered chain level. We finally induce the usual formulae for the equivariant cup and cap products from their analogs on the non-equivariant weight spectral sequences.

Comparison of stratified-algebraic and topological K-theory

Stratified-algebraic vector bundles on real algebraic varieties have many desirable features of algebraic vector bundles but are more flexible. We give a characterization of the compact real algebraic varieties X having the following property: There exists a positive integer r such that for any topological vector bundle ξ on X, the direct sum of r copies of ξ is isomorphic to a stratifiedalgebraic vector bundle. In particular, each compact real algebraic variety of dimension at most 8 has this property. Our results are expressed in terms of K-theory.

Real intersection homology II: A local duality obstruction

We prove that the intersection pairing on the real intersection homology of a real algebraic variety is not a dual pairing in general. A classical argument of Thom for manifolds, adapted to real intersection homology, shows that if this intersection pairing is nonsingular and X is the link of a point in a real algebraic variety, with the dimension of X even, then the intersection homology Euler characteristic Iχ(X) is even. Using an existence theorem of Akbulut and King [1] we show there is a singular algebraic surface X that is the link of a point in a 3-dimensional real algebraic variety, such that Iχ(X) is odd. Thus our definition of real intersection homology [6] does not have the key self-duality property suggested by Goresky and MacPherson ([3], p.227), though it does enjoy their small resolution property.

Computing convex hulls of trajectories

We study the convex hulls of trajectories of polynomial dynamical systems. Such trajectories include real algebraic curves. The boundaries of the resulting convex bodies are stratified into families of faces. We present numerical algorithms for identifying these patches. An implementation based on the software Bensolve Tools is given. This furnishes a key step in computing attainable regions of chemical reaction networks.

Separating semigroup of hyperelliptic curves and of genus 3 curves

A rational function on a real algebraic curve $C$ is called separating if it takes real values only at real points. Such a function defines a covering $\Bbb R C\to\Bbb{RP}^1$. Let $A_1,\dots,A_n$ be connected components of $C$. In a recent paper M. Kummer and K. Shaw defined the separating semigroup of $C$ as the set of all sequences $(d_1(f),\dots,d_n(f))$ where $f$ is a separating function and $d_i(f)$ is the degree of the restriction of $f$ to $A_i$. We describe the separating semigroup for hyperelliptic curves and for genus 3 curves.

Computing real radicals and S-radicals of polynomial systems

Let f=(f1,…,fs) be a sequence of polynomials in Q[X1,…,Xn] of maximal degree D and V⊂Cn be the algebraic set defined by f and r be its dimension. The real radical 〈f〉re associated to f is the largest ideal which defines the real trace of V. When V is smooth, we show that 〈f〉re, has a finite set of generators with degrees bounded by deg⁡V. Moreover, we present a probabilistic algorithm of complexity (snDn)O(1) to compute the minimal primes of 〈f〉re. When V is not smooth, we give a probabilistic algorithm of complexity sO(1)(nD)O(nr2r) to compute rational parametrizations for all irreducible components of the real algebraic set V∩Rn.Let (g1,…,gp) in Q[X1,…,Xn] and S be the basic closed semi-algebraic set defined by g1≥0,…,gp≥0. The S-radical of 〈f〉, which is denoted by 〈f〉S, is the ideal associated to the Zariski closure of V∩S. We give a probabilistic algorithm to compute rational parametrizations of all irreducible components of that Zariski closure, hence encoding 〈f〉S. Assuming now that D is the maximum of the degrees of the fi's and the gi's, this algorithm runs in time 2p(s+p)O(1)(nD)O(rn2r). Experiments are performed to illustrate and show the efficiency of our approaches on computing real radicals.

Blown-up Čech cohomology and Cartan’s Theorem B on real algebraic varieties

We introduce a concept of blown-up Čech cohomology for coherent sheaves of homological dimension [Formula: see text] and some quasi-coherent sheaves on a nonsingular real affine variety. Its construction involves a directed set of multi-blowups. We establish, in particular, long exact cohomology sequence and Cartan’s Theorem B. Finally, some applications are provided, including universal solution to the first Cousin problem (after blowing up).

Real root finding for low rank linear matrices

We consider $m \times s$ matrices (with $m\geq s$) in a real affine subspace of dimension $n$. The problem of finding elements of low rank in such spaces finds many applications in information and systems theory, where low rank is synonymous of structure and parsimony. We design computer algebra algorithms, based on advanced methods for polynomial system solving, to solve this problem efficiently and exactly: the input are the rational coefficients of the matrices spanning the affine subspace as well as the expected maximum rank, and the output is a rational parametrization encoding a finite set of points that intersects each connected component of the low rank real algebraic set. The complexity of our algorithm is studied thoroughly. It is polynomial in $\binom{n+m(s-r)}{n}$. It improves on the state-of-the-art in computer algebra and effective real algebraic geometry. Moreover, computer experiments show the practical efficiency of our approach.

On the rational function field of real curves without real points

Let F be the field of rational functions of a real algebraic curve without real points. It is well-known that in F we can express -1 as a sum of two squares. We show that -1 is also a sum of four fourth powers, six sixth powers, and so on.

Some examples of global Poisson structures on $S^4$

A Poisson structure is a bivector whose Schouten bracket vanishes. We study a global Poisson structure on $S^4$ associated with a holomorphic Poisson structure on $\mathbf{CP}^3$. The space of such Poisson structures on $S^4$ is realised as a real algebraic variety in the space of holomorphic Poisson structures on $\mathbf{CP}^3$. We generalize the result to the higher dimensional case $\mathbf{HP}^n$ by the twistor method. It is known that a holomorphic Poisson structure on $\mathbf{CP}^3$ corresponds to a codimension one holomorphic foliation and the space of these foliations of degree 2 has six components. In this paper we provide examples of Poisson structures on $S^4$ associated with these components.