hermann on Fri, 29 Aug 2025 21:56:12 +0200


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

On 2025-08-28 21:53, Karim Belabas wrote:


A generic solution:

? double_factorial(n) = vecprod(vector(n \ 2, i, n-2*i+2));
...
Cheers,

    K.B.


Thanks, that is fast and gives same result as oeis series:
https://oeis.org/search?q=5%2c7%2c13%2c113%20id:A118743

~$ gp -q
? double_factorial(n) = vecprod(vector(n \ 2, i, n-2*i+2));
? [s|n<-[1..250];s<-[double_factorial(n)+n+1],isprime(s)]
[3, 5, 7, 13, 113, 3851, 46093, 10321937, 185794579, 216862434431944426122117120047, 1874480713189654839282450157086195210900931486780353469173376681742503659434414632312033372643386765197475111737132341321962070865661132800000000000000000173] 
?

Regards,

Hermann.