Bill Allombert on Tue, 24 Feb 2009 00:06:25 +0100

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Re: ffinit

On Sun, Feb 22, 2009 at 07:08:16PM +0000, John Cremona wrote:
> I noticed that ffinit(3,582) produces a polynomial with 333 terms.  I
> would have thought that using a sparse polynomial as the modulus would
> be more efficient (I found x^582 + x^43 + x  - 1, for example).

PARI does not implement a reduction algorithm that is significantly
faster with sparse polynomial:
? P=ffinit(3,582);
? ffgen(P)^(3^582-1);
time = 396 ms.
? Q=(x^582 + x^43 + x - 1)*Mod(1,3);
? ffgen(Q)^(3^582-1);
time = 389 ms.

so the only efficiency considered here is the efficiency of computing the
polynomial itself and there is no contest:

? ffinit(3,582);
time = 12 ms.
? polisirreducible((x^582 + x^43 + x  - 1)*Mod(1,3));
time = 72 ms.
? for(i=2,581,P=x^582+x^i+x-1;if(polisirreducible(P*Mod(1,3)),print(P);break));
x^582 + x^43 + x - 1
time = 2,977 ms.

> I found the course code in polarit3.c which refers to various papers
> and algorithms (Lenstra and Adleman), but can anyone say what are the
> benefits of the polynomials these algorithms produce?  Actually I

1) The algorithm is deterministic so the output is well defined.
2) They have "small" coefficients (when centerlift()'ed).
3) They can be computed very quickly (in polynomial time assuming the

> could not quite tell whether in my case it might have been adding a
> random poly of degree < 582 to x^582, in which case the denseness of
> the reslt is not so surprising.

It does not. Actually in this example, it returns the compositum of the
finite fields defined by polsubcyclo(5,2), polsubcyclo(7, 3) and
polsubcyclo(389, 97) (which are all irreducible mod 3).