A271396 Integers k such that 3*k!!! - 1 is prime where k!!! is A007661(k).
0, 1, 2, 4, 5, 6, 7, 8, 10, 17, 25, 28, 31, 37, 38, 39, 46, 47, 49, 55, 67, 82, 85, 94, 98, 115, 120, 129, 167, 214, 216, 267, 293, 580, 732, 857, 993, 1012, 1069, 1308, 1430, 2366, 2974, 4017, 4870, 9034, 9061, 9752, 10657, 13847, 25390
Offset: 1
Examples
4 is a term because 3*4!!! - 1 = 11 is prime.
Programs
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Mathematica
Select[Range[0,5000],PrimeQ[3Times@@Range[#,1,-3]-1]&] (* The program generates the first 45 terms of the sequence. *) (* Harvey P. Dale, Mar 29 2025 *)
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PARI
is(k) = ispseudoprime(3*prod(i=0, (k-2)\3, k-3*i) - 1); \\ Jinyuan Wang, Jun 09 2021
Extensions
a(46)-a(50) from Jinyuan Wang, Jun 09 2021
a(51) from Michael S. Branicky, Aug 09 2024
Comments