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Primes of the form 2*k!! - 1.
2

%I #14 May 15 2021 11:48:44

%S 3,5,29,1889,20789,270269,15811707161249,443286190953399543749,

%T 433724868863888852244234239999,

%U 163685683629861106170483229851647999999,10144121732653119949813597183199268946575359999999,67476497990875549413061344119283906281177486718749

%N Primes of the form 2*k!! - 1.

%C The corresponding k are in A215779.

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

%t a={}; Do[p= 2*n!! -1; If[PrimeQ[p], AppendTo[a, p]], {n, 0,800}]; Print[a];

%t Select[2 Range[70]!!-1,PrimeQ] (* _Harvey P. Dale_, Dec 31 2020 *)

%Y Cf. A006882, A215779.

%K nonn

%O 1,1

%A _Michel Lagneau_, Aug 23 2012

%E Offset changed to 1 by _Jinyuan Wang_, May 15 2021