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Bill Allombert on Tue, 09 Sep 2025 10:09:50 +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.math.u-bordeaux.fr
- Subject: Re: PARI/GP wrong on 7!!+7+1 being not prime, or oeis.org/Mathematica?
- From: Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>
- Date: Tue, 9 Sep 2025 10:09:45 +0200
- Delivery-date: Tue, 09 Sep 2025 10:09:50 +0200
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- In-reply-to: <m3o6rz5m2t.fsf@lugabout.jhcloos.org>
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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