The Gcdex function is a placeholder for the extended Euclidean algorithm applied to a and b which are polynomials in x over a field.  Gcdex computes g, the greatest common divisor of a and b, which is a monic polynomial in x.  Additionally s and t are (if present) assigned polynomials in x such that    with  and . Gcdex is used in conjunction with either mod or evala as described below, both of which define the coefficient domain.

The call Gcdex(a, b, x, 's', 't') mod p performs the computation modulo p a prime integer. The multivariate polynomials a and b must have rational coefficients or coefficients in a finite field specified by RootOfs.

The call evala(Gcdex(a, b, x, 's', 't')) does likewise. The multivariate polynomials a and b must have algebraic number (or function) coefficients specified by RootOfs.