Double factorial and primorial

[Niels Möller]

bodrato at mail.dm.unipi.it writes:

> I implemented it, do you like the name mpz_fac2_ui?

Nice! Maybe it would be good with a slightly longer name, "2" is used
for many other things. Maybe "mpz_double_fac_ui" is better?

> I implemented the second one: "primes below n (included, if prime)", I
> called it mpz_primorial_ui. 50 lines of code recycling the structure built
> for mpz_fac_ui.

I wonder for how large n it is practical to count the number of primes
below n (without going to the trouble of multiplying them all together),
is at as difficult as factoring, or even worse?

/nisse

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