gcd denotes the gcd (greatest common divisor) of two polynomials (or of a list of polynomials or of a sequence of polynomials) (see also 5.6.2 for GCD of integers).
For polynomials with modular coefficients, input e.g. :
^2+2*x+1) mod 5,(x
^2-1) mod 5)
Note that :
^2-1) mod 5
will output :
since the mod operation is done after the GCD is computed in ℤ[X].