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 A215780 #14 May 15 2021 11:48:44
%S A215780 3,5,29,1889,20789,270269,15811707161249,443286190953399543749,
%T A215780 433724868863888852244234239999,
%U A215780 163685683629861106170483229851647999999,10144121732653119949813597183199268946575359999999,67476497990875549413061344119283906281177486718749
%N A215780 Primes of the form 2*k!! - 1.
%C A215780 The corresponding k are in A215779.
%e A215780 29 = 2*5!! - 1 = 2*15 - 1 is in the sequence.
%t A215780 a={}; Do[p= 2*n!! -1; If[PrimeQ[p], AppendTo[a, p]], {n, 0,800}]; Print[a];
%t A215780 Select[2 Range[70]!!-1,PrimeQ] (* _Harvey P. Dale_, Dec 31 2020 *)
%Y A215780 Cf. A006882, A215779.
%K A215780 nonn
%O A215780 1,1
%A A215780 _Michel Lagneau_, Aug 23 2012
%E A215780 Offset changed to 1 by _Jinyuan Wang_, May 15 2021