This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A217650 #18 Jul 26 2024 07:59:55
%S A217650 2,3,4,5,11,23,27,29,36,40,41,71,89,119,127,157,163,187,652,1374,1518,
%T A217650 2922,5193,6663,7455,9739,11569,14103
%N A217650 Numbers k such that 2*k!!! - 1 is prime.
%C A217650 k!!! is a triple factorial, see the definition in A007661.
%e A217650 5 is in the sequence because 2*5!!! - 1 = 2*10 - 1 = 19 is prime.
%p A217650 A:= n -> mul(k, k = select(k -> k mod 3 = n mod 3, [$1 .. n])): for p from 0 to 200 do:if type(2*A(p)-1,prime)=true then printf(`%d, `,p):else fi:od:
%t A217650 lst={}; multiFactorial[n_, k_] := If[n < 1, 1, If[n < k + 1, n, n*multiFactorial[n - k, k]]]; Do[If[PrimeQ[2*multiFactorial[n, 3] - 1], AppendTo[lst, n]], {n, 0, 1000}]; lst
%o A217650 (PARI) is(n)=ispseudoprime(2*prod(i=0, (n-2)\3, n-3*i)-1) \\ _Charles R Greathouse IV_, Oct 09 2012
%o A217650 (PFGW)
%o A217650 ABC2 2*$a!3-1
%o A217650 a: from 1 to 6000
%o A217650 _Charles R Greathouse IV_, Oct 09 2012
%Y A217650 Cf. A007661, A217647, A217650.
%K A217650 nonn,hard,more
%O A217650 1,1
%A A217650 _Michel Lagneau_, Oct 09 2012
%E A217650 a(20)-a(23) from _Charles R Greathouse IV_, Oct 09 2012
%E A217650 a(24)-a(25) from _Jinyuan Wang_, May 15 2021
%E A217650 a(26)-a(28) from _Michael S. Branicky_, Jul 25 2024