minimal support for multivariate factorisation in PARI

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

We have added minimal support for multivariate factorisation of
polynomials to the factor function.
The implementation is very inefficient, this is meant for case
where software better suited for that task are not available.
It works for all domains that are supported by factor, tought
it work better over Z or over a number field.

We do not have plan to improve the implementation.

This is some examples:

factor((x^3+y^3)^2*(x^4+y^4))
factor(x^3+y^3)
factor((x^7-y^7)*Mod(1,7))
factor((x^7-y)*Mod(1,7))
factor(x^5+y*x^4+y^2*x^3+(y^2-1)*x^2+(y^3-y)*x+(y^4-y^2))
factor((x^2+y^2) * Mod(1,13))
factor(x^3+I*y*x^2+(y^2+I)*x+(I*y^3-y))
factor((x^3+y^3+z^3-3*x*y*z)*Mod(1,7))
factor((x^3+y^3+z^3-3*x*y*z)*Mod(1,a^2+3))
factor(x*y*(x+y))

E(n)=ellinit([2,1-n^2,(1-n^2)*(1+n),0,0]);
factor(elldivpol(ellinit(E(t)),4))

Cheers,
Bill.