## Section: Scientific Foundations

### Zero–dimensional systems

Participants : L. Bettale, J.C. Faugère, D. Lazard, C. Mou, J. Svartz, P.J. Spaenlehauer.

A system is zero-dimensional if the set of the solutions in an algebraically closed field is finite. In this case, the set of solutions does not depend on the chosen algebraically closed field.

Such a situation can easily be detected on a Gröbner basis for any admissible monomial ordering.

These systems are mathematically particular since one can systematically bring them back to linear algebra problems. More precisely, the algebra $K[{X}_{1},...,{X}_{n}]/I$ is in fact a $K$-vector space of dimension equal to the number of complex roots of the system (counted with multiplicities). We chose to exploit this structure. Accordingly, computing a base of $K[{X}_{1},...,{X}_{n}]/I$ is essential. A Gröbner basis gives a canonical projection from $K[{X}_{1},...,{X}_{n}]$ to $K[{X}_{1},...,{X}_{n}]/I$, and thus provides a base of the quotient algebra and many other informations more or less straightforwardly (number of complex roots for example).

The use of this vector-space structure is well known and at the origin of the one of the most known algorithms of the field ( [49] ) : it allows to deduce, starting from a Gröbner basis for any ordering, a Gröbner base for any other ordering (in practice, a lexicographic basis, which are very difficult to compute directly). It is also common to certain semi-numerical methods since it allows to obtain quite simply (by a computation of eigenvalues for example) the numerical approximation of the solutions (this type of algorithms is developed, for example, in the INRIA Galaad project).

Contrary to what is written in a certain literature, the computation of Gröbner bases is not "doubly exponential" for all the classes of problems. In the case of the zero-dimensional systems, it is even shown that it is simply exponential in the number of variables, for a degree ordering and for the systems without zeros at infinity. Thus, an effective strategy consists in computing a Gröbner basis for a favorable ordering and then to deduce, by linear algebra technics, a Gröbner base for a lexicographic ordering [49] .

The case of the zero-dimensional systems is also specific for triangular sets. Indeed, in this particular case, we have designed algorithms that allow to compute them efficiently [59] starting from a lexicographic Gröbner basis. Note that, in the case of zero-dimensional systems, regular triangular sets are Gröbner bases for a lexicographical order.

Many teams work on Gröbner bases and some use triangular sets in the case of the zero-dimensional systems, but up to our knowledge, very few continue the work until a numerical resolution and even less tackle the specific problem of computing the real roots. It is illusory, in practice, to hope to obtain numerically and in a reliable way a numerical approximation of the solutions straightforwardly from a lexicographical basis and even from a triangular set. This is mainly due to the size of the coefficients in the result (rational number).

The use of innovative algorithms for Gröbner bases computations [4] , [3] , Rational Univariate representations ( [49] and [38] for the "shape position" case, allows to use zero-dimensional solving as sub-task in other algorithms.