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 A118257 #19 Dec 12 2024 04:45:21
%S A118257 2,6,8,10,18,26,30,74,142,203,398,651,792,1314,3487,5978,6240,7814,
%T A118257 8054,8673,21436,23947,52985,91784,157537,164901
%N A118257 Numbers k such that A118255(k) is prime.
%C A118257 A118255(1314) is prime with 396 digits
%C A118257 A118255(23947) is a probable prime with 7209 digits. - _Giovanni Resta_, Apr 26 2006
%e A118257 A118255(2) = 2 prime, A118255(6) = 149 prime, A118255(8) = 599 prime.
%t A118257 A118255[n_] := Module[{},
%t A118257 If[n == 1, A118255[1] = 1,
%t A118257 If[PrimeQ[n], A118255[n] = 2 A118255[n - 1],
%t A118257 A118255[n] = 2 A118255[n - 1] + 1]]];
%t A118257 Select[Range[5000], PrimeQ[A118255[#]] &] (* _Robert Price_, Apr 03 2019 *)
%Y A118257 Cf. A005171, A118255, A118256.
%K A118257 nonn,more
%O A118257 1,1
%A A118257 _Pierre CAMI_, Apr 19 2006
%E A118257 a(15)-a(22) from _Giovanni Resta_, Apr 26 2006
%E A118257 a(23)-a(26) from _Michael S. Branicky_, Dec 11 2024