### 2.31.13 GCD in ℤ/pℤ[x] : `gcd`

`gcd` takes as arguments two polynomials with coefficients in ℤ/pℤ (p must be prime).
`gcd` returns the GCD of these polynomials computed in ℤ/pℤ[x] (see also 2.25.7 for polynomials with non modular coefficients).
Input :

`gcd((2*x```^```2+5)%13,(5*x```^```2+2*x-3)%13)`

Output :

`(-4%13)*x+5%13`

Input :

`gcd((x```^```2+2*x+1,x```^```2-1)) mod 5)`

Output :

`x%5`

Note the difference with a gcd computation in ℤ[X] followed by a reduction modulo 5, input:

`gcd(x```^```2+2*x+1,x```^```2-1) mod 5`

Output :

`1`