Re: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?

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

On Thu, Aug 28, 2025 at 02:37:46PM -0400, James Cloos wrote:
> BA> Adding this one would break backward compatibility, so this is not
> BA> an option.
> 
> does pari have a generalized factorial function?
> 
> maxima's doc notes this:
> 
>      For an argument ‘n’ which is not an integer, float, or rational,
>      ‘n!!’ yields a noun form ‘genfact (n, n/2, 2)’.
> 
> perhaps pari's docs could suggest using such a generalized factorial
> when a double factorial is desired?

You can use
doublefact(n)=vecprod([2*i-1|i<-[1..(n+1)\2]])

Cheers,
Bill