Re: Proposal to extend primes()

[Karim Belabas <Karim.Belabas@math.u-bordeaux.fr>]

* 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
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