Elisabetta Fortuna

Algorithms to compute the topology of orientable real algebraic surfaces

We present constructive algorithms to determine the topological type of a non-singular orientable real algebraic projective surface S in the real projective space, starting from a polynomial equation with rational coefficients for S. We address this question when there exists a line in RP3 not intersecting the surface, which is a decidable problem; in the case of quartic surfaces, when this condition is always fulfilled, we give a procedure to find a line disjoint from the surface. Our algorithm computes the homology of the various connected components of the surface in a finite number of steps, using as a basic tool Morse theory. The entire procedure has been implemented in Axiom.

Degree reduction under specialization

We examine the degree relationship between the elements of an ideal I ⊆ R[x] and the elements of ’(I ) where ’ : R → R is a ring homomorphism. When R is a multivariate polynomial ring over a 3eld, we use this relationship to show that the image of a Gr4 obner basis remains a Gr4 obner basis if we specialize all the variables but one, with no requirement on the dimension of I . As a corollary we obtain the GCD for a collection of parametric univariate polynomials. We also apply this result to solve parametric systems of polynomial equations and to reexamine the extension theoremfor such system s. c

Algorithmical determination of the topology of a real algebraic surface

We present an algorithm to compute the topology of a non-singular real algebraic surface S in RP^3, that is the number of its connected components and a topological model for each of them. Our strategy consists in computing the Euler characteristic of each connected component by means of a Morse-type investigation of S or of a suitably constructed compact affine surface. This procedure can be used to determine the topological type of an arbitrary non-singular surface; in particular it extends an existing algorithm applicable only to surfaces disjoint from a line.

Computing the topology of real algebraic surfaces

We present constructive algorithms to determine the topological type of a non-singular real algebraic projective surface S in the real projective space; we address this question when there exists a line in RP3 not intersecting the surface. Starting from a polynomial equation with rational coefficients for S, our algorithm computes the homology of the various connected components of the surface. We reconstruct the homology in a finite number of steps, using as a basic tool Morse theory. The entire procedure has been implemented in Axiom.

Local algebraic approximation of semianalytic sets

Abstract. Two subanalytic subsets of Rn are called s-equivalent at a common point P if the Hausdorff distance between their intersections with the sphere centered at P of radius r vanishes to order > s when r tends to 0. In this paper we prove that every s-equivalence class of a closed semianalytic set contains a semialgebraic representative of the same dimension. In other words any semianalytic set can be locally approximated of any order s by means of a semialgebraic set and hence, by previous results, also by means of an algebraic one

Algebraic approximation of germs of real analytic sets

Two subanalytic subsets of ℝ n are s-equivalent at a common point, say O, if the Hausdorff distance between their intersections with the sphere centered at O of radius r goes to zero faster than r s . In the present paper we investigate the existence of an algebraic representative in every s-equivalence class of subanalytic sets. First we prove that such a result holds for the zero-set V(f) of an analytic map f when the regular points of f are dense in V(f). Moreover we present some results concerning the algebraic approximation of the image of a real analytic map f under the hypothesis that f ―1 (O) = {O} . .

Generators of the ideal of an algebraic space curve

In this paper we show that the ideal of any algebraic curve in affine 3-space whose Jacobian matrix has rank at least 1 at every singular point of the curve can be generated by three polynomials and we give constructive procedures to compute such generators.

Some constructions for real algebraic curves

We address two basic questions for real algebraic curves. The first one is how to decide whether a real algebraic curve in the n-projective space contains some real point. We present an algorithm that reduces the original question to deciding whether the zero-set of a zero-dimensional ideal contains real points. The second part of the paper is devoted to giving necessary and sufficient conditions for the existence of a real line disjoint from a given real plane algebraic curve. An algorithm for testing whether these conditions are fulfilled is given. are fulfilled is given.

A separation theorem in dimension

Let M be a compact non-singular real affine algebraic variety and let A, B be open disjoint semialgebraic subsets of M . Define (where —4 denotes the Zariski closure).

3

The paper deals with the question of recognizing the mutual positions of the connected components of a non-singular real projective surface S in the real projective 3-space. We present an algorithm that answers this question through the computation of the adjacency graph of the surface; it also allows to decide whether each connected component is contractible or not. The algorithm, combined with a previous one returning as an output the topology of the surface, computes a set of data invariant up to ambient-homeomorphism which, though not sufficient to determine the pair , give information about the nature of the surface as an embedded object.