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

On 2025-08-28 13:48, [email protected] 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