Re: Proposal to extend primes()

[John Cremona <john.cremona@gmail.com>]

Wow, gp syntax is getting quite sophisticated these days!

John

On 16 March 2016 at 12:16, Karim Belabas
<Karim.Belabas@math.u-bordeaux.fr> wrote:
> * John Cremona [2016-03-16 12:48]:
>> DIfferent people will want different abbreviations for their common
>> use case.  I can imagine wanting (for example) primes(1,4) to mean
>> primes congruent to 1 mod 4 (and then another parameter needed for a
>> bound).  I can also imagine a *very* common use for primes(pmax, N)
>> returning the primes up to pmax which do not divide N.
>
> An unrelated note: Comprehension already allows to express this in
> a natural way:
>
>   [ p | p <- primes([2,1000]), N % p != 0 ]
>
> N.B. And so would
>
>   [ n | n <- [2..1000], isprime(n) && N % n ]
>
> without bothering about primes() and how to pass its arguments, but the
> call to isprime() in this latter construction is a major loss of
> efficiency. (While I see no way to produce primes coprime to N in a
> significantly faster way than the first algorithm.)
>
> Cheers,
>
>     K.B.
> --
> Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
> Universite de Bordeaux         Fax: (+33) (0)5 40 00 69 50
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