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 A271396 #23 Mar 29 2025 18:43:10 %S A271396 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, %T A271396 115,120,129,167,214,216,267,293,580,732,857,993,1012,1069,1308,1430, %U A271396 2366,2974,4017,4870,9034,9061,9752,10657,13847,25390 %N A271396 Integers k such that 3*k!!! - 1 is prime where k!!! is A007661(k). %C A271396 Corresponding primes are 2, 2, 5, 11, 29, 53, 83, 239, 839, 628319, 1825823999, 51123071999, ... %e A271396 4 is a term because 3*4!!! - 1 = 11 is prime. %t A271396 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 *) %o A271396 (PARI) is(k) = ispseudoprime(3*prod(i=0, (k-2)\3, k-3*i) - 1); \\ _Jinyuan Wang_, Jun 09 2021 %Y A271396 Cf. A007661, A037083, A084438, A217647, A215779, A271392. %K A271396 nonn,more %O A271396 1,3 %A A271396 _Altug Alkan_, Apr 06 2016 %E A271396 a(46)-a(50) from _Jinyuan Wang_, Jun 09 2021 %E A271396 a(51) from _Michael S. Branicky_, Aug 09 2024