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

[Denis Simon <denis.simon@unicaen.fr>]

Another possibility is to add Bill's suggestion in the help ?factorial or ??factorial.

Denis SIMON.

----- Mail original -----
> De: "Bill Allombert" <Bill.Allombert@math.u-bordeaux.fr>
> À: "pari-users" <pari-users@pari.math.u-bordeaux.fr>
> Envoyé: Mardi 9 Septembre 2025 10:09:45
> Objet: Re: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?

> 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