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Denis Simon on Tue, 09 Sep 2025 14:03:53 +0200
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Re: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?
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- To: pari-users <pari-users@pari.math.u-bordeaux.fr>
- Subject: Re: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?
- From: Denis Simon <denis.simon@unicaen.fr>
- Date: Tue, 9 Sep 2025 14:03:49 +0200 (CEST)
- Cc: Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>
- Delivery-date: Tue, 09 Sep 2025 14:03:53 +0200
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- Thread-topic: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?
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