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Ruud H.G. van Tol on Thu, 28 Aug 2025 14:15:38 +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: "Ruud H.G. van Tol" <rvtol@isolution.nl>
- Date: Thu, 28 Aug 2025 14:15:32 +0200
- Delivery-date: Thu, 28 Aug 2025 14:15:38 +0200
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- In-reply-to: <147f7756bbcb46d77e1c67c39d8aeedd@stamm-wilbrandt.de>
- References: <147f7756bbcb46d77e1c67c39d8aeedd@stamm-wilbrandt.de>
On 2025-08-28 13:48, hermann@stamm-wilbrandt.de wrote:
The sequence is:
https://oeis.org/search?q=5%2c7%2c13%2c113%20id:A118743
[...]
Is 7!!+7+1 prime or not?
hermann@7950x:~$ gp -q
? for(n=1,12,if(isprime(n!!+n+1),print(n,": ",n!!+n+1));print1("."))
1: 3
.2: 5
...........
? ##
*** last result computed in 4min, 32,167 ms.
If !! means "double factorial", then 1 is the only odd that can lead to
a prime:
3!! is even, plus 3 plus 1, is still even.
So what does !! mean here?
(see also the Maple code)
-- Ruud