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 A215779 #20 Aug 09 2024 13:53:43
%S A215779 2,3,5,9,11,13,25,35,46,56,68,69,71,84,92,103,110,121,193,259,308,368,
%T A215779 406,426,620,627,1206,1631,1845,2108,2210,2786,7931,8008,8367,12181,
%U A215779 12291
%N A215779 Numbers k such that 2*k!! - 1 is prime.
%C A215779 The corresponding primes are in A215780.
%e A215779 5 is in the sequence because 2*5!! - 1 = 2*15 - 1 = 29 is a prime.
%t A215779 lst={};Do[If[PrimeQ[2*n!!-1],AppendTo[lst,n]],{n,0,1500}];lst
%Y A215779 Cf. A006882, A215780.
%K A215779 nonn,hard,more
%O A215779 1,1
%A A215779 _Michel Lagneau_, Aug 23 2012
%E A215779 Offset changed to 1 by and a(27)-a(32) from _Jinyuan Wang_, May 15 2021
%E A215779 a(33)-a(37) from _Michael S. Branicky_, Aug 09 2024