A new numerical method for the computation of the greatest common divisor (GCD) of an m-set of polynomials of lR[s], P m • d of maximal degree d, is presented. This method is based on a recently developed theoretical algorithm (Karcanias 1987) that uses elementary transformations and shifting operations; the present algorithm takes into account the non-generic nature of GCD and thus uses steps, which minimize the introduction of additional errors and defines the GCD in an approximate sense. For a given set Pm,d, with a basis matrix Pm, the method defines first, the most orthogonal uncorrupted base P, from the rows of P«, where r = rank (Pm) '" m. By applying successively gaussian transformations and shifting, on the basis matrix P, E R,x(d+ll, we produce each time a new basis matrix P, with z = rank (P,) < r. The method terminates when the rank of P z is approximately equal to 1; the coefficient vector of the GCD is then defined as a row of the unit rank matrix P,. The method defines the exact degree of the GCD, successfully evaluates an approximate solution and works satisfactorily with large numbers of polynomials of any fixed degree.

The computation of the greatest common divisor (GCD) of many polynomials is a nongeneric problem. Techniques defining "approximate GCD" solutions have been defined, but the proper definition of the "approximate" GCD, and the way we can measure the strength of the approximation has remained open. This paper uses recent results on the representation of the GCD of many polynomials, in terms of factorisation of generalised resultants, to define the notion of "approximate GCD" and define the strength of any given approximation by solving an optimisation problem. The newly established framework is used to evaluate the performance of alternative procedures which have been used for defining approximate GCDs. (~)

The computation of the greatest common divisor (GCD) of a set of polynomials has interested the mathematicians for a long time and has attracted a lot of attention in recent years. A challenging problem that arises from several applications, such as control or image and signal processing, is to develop a numerical GCD method that inherently has the potential to work efficiently with sets of several polynomials with inexactly known coefficients. The presented work focuses on : (i) the use of the basic principles of the ERES methodology for calculating the GCD of a set of several polynomials and defining approximate solutions by developing the hybrid implementation of this methodology. (ii) the use of the developed framework for defining the approximate notions for the GCD as a distance problem in a projective space to develop an optimization algorithm for evaluating the strength of different ad-hoc approximations derived from different algorithms. The presented new implementation of ERES is based on the effective combination of symbolic-numeric arithmetic (hybrid arithmetic) and shows interesting computational properties for the approximate GCD problem. Additionally, an efficient implementation of the strength of an approximate GCD is given by exploiting some of the special aspects of the respective distance problem. Finally, the overall performance of the ERES algorithm for computing approximate solutions is discussed.

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