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Generalized polar varieties: geometry and algorithms. (English) Zbl 1085.14047

Summary: Let \(V\) be a closed algebraic subvariety of the \(n\)-dimensional projective space over the complex or real numbers and suppose that \(V\) is non-empty and equidimensional. The classic notion of a polar variety of \(V\) associated with a given linear subvariety of the ambient space of \(V\) was generalized and motivated by the authors in [Generalized polar varieties and an efficient real elimination procedure, Kybernetika, 40 (5), 519–550 (2004)]. As particular instances of this notion of a generalized polar variety one reobtains the classic one and an alternative type of a polar variety, called dual. As main result of the present paper we show that for a generic choice of their parameters the generalized polar varieties of \(V\) are empty or equidimensional and smooth in any regular point of \(V\). In the case that the variety \(V\) is affine and smooth and has a complete intersection ideal of definition, we are able, for a generic parameter choice, to describe locally the generalized polar varieties of \(V\) by explicit equations. Finally, we indicate how this description may be used in order to design in the context of algorithmic elimination theory a highly efficient, probabilistic elimination procedure for the following task: In case, that the variety \(V\) is \(\mathbb Q\)-definable and affine, having a complete intersection ideal of definition, and that the real trace of \(V\) is non-empty and smooth, find for each connected component of the real trace of \(V\) an algebraic sample point.

MSC:

14P05 Real algebraic sets
14B05 Singularities in algebraic geometry
14Q99 Computational aspects in algebraic geometry
68Q25 Analysis of algorithms and problem complexity
68W30 Symbolic computation and algebraic computation

Software:

Kronecker
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