\( \newcommand{\E}{\mathrm{E}} \) \( \newcommand{\A}{\mathrm{A}} \) \( \newcommand{\R}{\mathrm{R}} \) \( \newcommand{\N}{\mathrm{N}} \) \( \newcommand{\Q}{\mathrm{Q}} \) \( \newcommand{\Z}{\mathrm{Z}} \) \( \def\ccSum #1#2#3{ \sum_{#1}^{#2}{#3} } \def\ccProd #1#2#3{ \sum_{#1}^{#2}{#3} }\)
CGAL 4.4 - Polynomial
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PolynomialTraits_d::Resultant Concept Reference

Definition

This AdaptableBinaryFunction computes the resultant of two polynomials \( f\) and \( g\) of type PolynomialTraits_d::Polynomial_d with respect to a certain variable.

Note that this functor operates on the polynomial in the univariate view, that is, the polynomial is considered as a univariate polynomial in one specific variable.

Let \( f\) and \( g\) be two univariate polynomials over some commutative ring \( A\), where

\[ f = f_mx^m + \dots + f_0 \]

and

\[ g = g_nx^n + \dots + g_0. \]

The resultant of \( f\) and \( g\) is defined as the determinant of the Sylvester matrix:

sylvester_matrix.png

Note that this is a \( (n+m)\times(n+m)\) matrix as there are \( n\) rows for \( f\) and \( m\) rows that are used for \( g\). The blank spaces are supposed to be filled with zeros.

Advanced

Let \( L\) be the algebraic closure of \( A\), and write \( f\) and \( g\) as

\[ f := f_m \ccProd{i=1}{m}{(x-\alpha_i)},\ \alpha_i \in L \]

and

\[ g := g_n \ccProd{j=1}{n}{(x-\beta_j)},\ \beta_i \in L, \]

then the resultant of \( f\) and \( g\) is (up to leading coefficients) the product of all pairwise differences of the roots of \( f\) and \( g\), namely

\[ res(f,g) = f_m^n g_n^m \ccProd{i=1}{m}{\ccProd{j=1}{n}{(\alpha_i-\beta_j)}}. \]

In particular, \( res(f,g) \neq 0\) iff \( f\) and \( g\) have a common factor with a positive degree in \( X\).

There are various ways to compute the resultant. Naive options are the computation of the resultant as the determinant of the Sylvester Matrix or the Bezout Matrix as well as the so called subresultant algorithm, which is a variant of the Euclidean Algorithm. More sophisticated methods may use modular arithmetic and interpolation. For more information we refer to, e.g., [2].

Refines:

AdaptableBinaryFunction

CopyConstructible

DefaultConstructible

See Also
Polynomial_d
PolynomialTraits_d
PolynomialTraits_d::UnivariateContent
PolynomialTraits_d::PolynomialSubresultants
PolynomialTraits_d::PrincipalSubresultants

Types

typedef
PolynomialTraits_d::Coefficient_type 
result_type
 
typedef
PolynomialTraits_d::Polynomial_d 
first_argument_type
 
typedef
PolynomialTraits_d::Polynomial_d 
second_argument_type
 

Operations

result_type operator() (first_argument_type f, second_argument_type g)
 Computes the resultant of \( f\) and \( g\), with respect to the outermost variable.