On the complexity of real solving bivariate systems

• Authors: Dimitrios I. Diochnos;Ioannis Z. Emiris;Elias P. Tsigaridas
• Venue: Proceedings of the 2007 international symposium on Symbolic and algebraic computation
• Year: 2007

We consider exact real solving of well-constrained, bivariate systems of relatively prime polynomials. The main problem is to compute all common real roots in isolating interval representation, and to determine their intersection multiplicities. We present three algorithms and analyze their asymptotic bit complexity, obtaining a bound of ÕB(N14) for the purely projection-based method, and ÕB(N12) for two subresultants-based methods: these ignore polylogarithmic factors, and N bounds the degree and the bitsize of the polynomials. The previous record bound was ÕB(N14). Our main tool is signed subresultant sequences, extended to several variables by binary segmentation. We exploit advances on the complexity of univariate root isolation, and extend them to multipoint sign evaluation, sign evaluation of bivariate polynomials over two algebraic numbers, and real root counting over an extension field. Our algorithms apply to the problem of simultaneous inequalities; they also compute the topology of real plane algebraic curves in. All algorithms have been implemented in maple, in conjunction with numeric filtering. We compare them against fgb/rs and synaps; we also consider maple libraries insulate and top, which compute curve topology. Our software is among the most robust, and its runtimes are within a small constant factor, with respect to the C/C++ libraries.