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Integers k such that 3*k!!! - 1 is prime where k!!! is A007661(k).
1

%I #21 Aug 10 2024 01:30:55

%S 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 115,120,129,167,214,216,267,293,580,732,857,993,1012,1069,1308,1430,

%U 2366,2974,4017,4870,9034,9061,9752,10657,13847,25390

%N Integers k such that 3*k!!! - 1 is prime where k!!! is A007661(k).

%C Corresponding primes are 2, 2, 5, 11, 29, 53, 83, 239, 839, 628319, 1825823999, 51123071999, ...

%e 4 is a term because 3*4!!! - 1 = 11 is prime.

%o (PARI) is(k) = ispseudoprime(3*prod(i=0, (k-2)\3, k-3*i) - 1); \\ _Jinyuan Wang_, Jun 09 2021

%Y Cf. A007661, A037083, A084438, A217647, A215779, A271392.

%K nonn,more

%O 1,3

%A _Altug Alkan_, Apr 06 2016

%E a(46)-a(50) from _Jinyuan Wang_, Jun 09 2021

%E a(51) from _Michael S. Branicky_, Aug 09 2024