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On using Lazard’s projection in CAD construction. (English) Zbl 1325.13026
The concept of cylindrical algebraic decomposition (CAD) of the Euclidean $$n$$-space $$E^n$$ has been introduced by G. E. Collins in [Lect. Notes Comput. Sci. 33, 134–183 (1975; Zbl 0318.02051)]. It is closely related to the classical simplicial and CW-complexes of algebraic topology and has several applications. Given a finite set $$A$$ of $$n$$-variate integral polynomials, a CAD is $$A$$-invariant if the polynomials of $$A$$ are sign-invariant in each of the regions of the decomposition. The definition of CAD is based on a sort of induction on the dimension of the Euclidean space and a key point for $$A$$-invariant CAD construction is the choice of a projection of the polynomials of $$A$$, i.e. a certain set of $$(n-1)$$-variate integral polynomials.
This paper is devoted to the investigation of $$A$$-invariant CAD construction focusing on the projection proposed by D. Lazard [in: Algebraic geometry and its applications. Collections of papers from Shreeram S. Abhyankar’s 60th birthday conference held at Purdue University, West Lafayette, IN, USA, June 1-4, 1990. New York: Springer-Verlag. 467–476 (1994; Zbl 0822.68118)], whose proof presented some flaws. Despite these flaws, the authors show that Lazard’s projection $$P_L(A)$$ is valid when the set $$A$$ is well-oriented with respect to $$P_L(A)$$ (see Definition 3.4).
In Section 2, the authors analyze the flaws of Lazard’s projection and compare it with the projection introduced by S. McCallum [J. Symb. Comput. 5, No. 1–2, 141–161 (1988; Zbl 0648.68054); in: Quantifier elimination and cylindrical algebraic decomposition. Proceedings of a symposium, Linz, Austria, October 6–8, 1993. Wien: Springer. 242–268 (1998; Zbl 0900.68279)] and with the Brown-McCallum projection [C. W. Brown, J. Symb. Comput. 32, No. 5, 447–465 (2001; Zbl 0981.68186)], motivating the interest for a reinstatement of Lazard’s method with several arguments (e.g., see Remark 3.2 and Section 4).
Section 3 contains the main result (Theorem 3.1) which leads to the development of a CAD algorithm using Lazard’s projection (Section 3.2). The rest of Section 3 is devoted to the proof of the main result, especially to the study of a technical lemma (Lemma 3.12) obtained as a consequence of an important theorem of Abhyankar and Jung to which a final Appendix is also devoted.

##### MSC:
 13P05 Polynomials, factorization in commutative rings 68W30 Symbolic computation and algebraic computation 03G15 Cylindric and polyadic algebras; relation algebras 14Q99 Computational aspects in algebraic geometry
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