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%I #16 May 15 2021 11:48:06
%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
%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
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