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 A181742 #10 Oct 16 2024 16:35:42
%S A181742 3,3,4,4,4,5,5,6,6,6,6,8,8,8,8,8,9,9,9,9,10,10,10,11,12,12,12,12,12,
%T A181742 13,14,14,14,14,14,16,16,16,17,17,18,18,18,18,18,18,18,18,18,19,20,20,
%U A181742 20,20,20,20,20,20,20,20,20,20,21,21,21,21,22,22,22,22,22,24,24,24,24,24,24,24
%N A181742 Let A181741(n)=2^(t(n))-2^(k(n))-1, where k(n)>=1, t(n)>=k(n)+1. Then a(n)=t(n).
%t A181742 f[n_] := IntegerExponent[n + 2^IntegerExponent[n, 2], 2]; f/@ (Select[Table[2^t-2^k-1, {t, 1, 20}, {k, 1, t-1}] // Flatten // Union, PrimeQ] + 1) (* _Amiram Eldar_, Dec 17 2018 after _Jean-François Alcover_ at A181741 *)
%o A181742 (PARI) listt(nn) = {for (n=3, nn, forstep(k=n-1, 1, -1, if (isprime(2^n-2^k-1), print1(n, ", "));););} \\ _Michel Marcus_, Dec 17 2018
%Y A181742 Cf. A181741, A181743.
%K A181742 nonn
%O A181742 1,1
%A A181742 _Vladimir Shevelev_, Nov 08 2010
%E A181742 Corrected and extended by _Michel Marcus_, Dec 17 2018