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Numbers k such that 2*k!! - 1 is prime.
3

%I #20 Aug 09 2024 13:53:43

%S 2,3,5,9,11,13,25,35,46,56,68,69,71,84,92,103,110,121,193,259,308,368,

%T 406,426,620,627,1206,1631,1845,2108,2210,2786,7931,8008,8367,12181,

%U 12291

%N Numbers k such that 2*k!! - 1 is prime.

%C The corresponding primes are in A215780.

%e 5 is in the sequence because 2*5!! - 1 = 2*15 - 1 = 29 is a prime.

%t lst={};Do[If[PrimeQ[2*n!!-1],AppendTo[lst,n]],{n,0,1500}];lst

%Y Cf. A006882, A215780.

%K nonn,hard,more

%O 1,1

%A _Michel Lagneau_, Aug 23 2012

%E Offset changed to 1 by and a(27)-a(32) from _Jinyuan Wang_, May 15 2021

%E a(33)-a(37) from _Michael S. Branicky_, Aug 09 2024