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 A136308 #27 Mar 27 2025 21:31:30 %S A136308 1,11,1111,11111111,1111111111111111,11111111111111111111111111111111, %T A136308 1111111111111111111111111111111111111111111111111111111111111111 %N A136308 a(n) = (10^2^n - 1)/9. %C A136308 More generally, reading in base B >= 2: a(n) = (B^2^n - 1)/(B-1). %C A136308 Recurrence: a(n) = a(n-1)*(B^K + 1) and a(0)=1 where K = floor(log_B a(n-1)) + 1. %C A136308 B = 2 gives A051179; B = 3 gives A059918. %F A136308 a(n) = a(n-1)*(10^K + 1) and a(0)=1 where K=floor(log_10 a(n-1)) + 1 = 2^n + 1. %F A136308 a(n) = A000042(A000079(n)) = A007088(A051179(n)) = A007089(A059918(n)). %F A136308 A007953(a(n)) = 2^n. - _Stefano Spezia_, Mar 27 2025 %t A136308 (10^2^Range[0, 10] - 1)/9 (* _G. C. Greubel_, Apr 19 2021 *) %o A136308 (Magma) A136308 := func<n|(10^2^n-1)/9>; [A136308(n):n in[0..7]]; %o A136308 (Sage) [(10^2^n -1)/9 for n in (0..10)] # _G. C. Greubel_, Apr 19 2021 %Y A136308 Cf. A000042 (repunits). %Y A136308 In other bases: A051179, A059918. %Y A136308 Cf. A000079, A007088, A007089, A007953. %K A136308 base,easy,nonn %O A136308 0,2 %A A136308 _Ctibor O. Zizka_, Mar 22 2008 %E A136308 Edited by _Jason Kimberley_, Dec 18 2012