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 A138826 #18 Aug 08 2024 16:18:23
%S A138826 101,11100111,1111100011111,111111100001111111,
%T A138826 11111111100000111111111,1111111111100000011111111111,
%U A138826 111111111111100000001111111111111,11111111111111100000000111111111111111
%N A138826 Concatenation of 2n-1 digits 1, n digits 0 and 2n-1 digits 1.
%C A138826 a(n) has 5n-2 digits.
%C A138826 a(n) is also A147540(n) written in base 2. [_Omar E. Pol_, Nov 08 2008]
%H A138826 Paolo Xausa, <a href="/A138826/b138826.txt">Table of n, a(n) for n = 1..195</a>
%H A138826 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (101101,-110201100,10110100000,-10000000000).
%F A138826 a(n) = (10^(2n-1)-1+10^(5n-2)-10^(3n-1))/9. [_R. J. Mathar_, Nov 07 2008, corrected Nov 09 2008]
%F A138826 G.f.: x*(1100000000*x^3-2000000*x^2+888910*x+101) / ((x-1)*(100*x-1)*(1000*x-1)*(100000*x-1)). - _Colin Barker_, Sep 16 2013
%e A138826 n ........... a(n)
%e A138826 1 ........... 101
%e A138826 2 ......... 11100111
%e A138826 3 ....... 1111100011111
%e A138826 4 ..... 111111100001111111
%e A138826 5 ... 11111111100000111111111
%t A138826 Table[(1000^n + 10)*(100^n - 10)/900, {n, 10}] (* _Paolo Xausa_, Aug 08 2024 *)
%o A138826 (PARI) Vec(x*(1100000000*x^3-2000000*x^2+888910*x+101)/((x-1)*(100*x-1)*(1000*x-1)*(100000*x-1)) + O(x^100)) \\ _Colin Barker_, Sep 16 2013
%Y A138826 Cf. A138120, A138144, A138145, A138146, A138148, A138720, A138721, A138722, A147540.
%K A138826 base,easy,nonn
%O A138826 1,1
%A A138826 _Omar E. Pol_, Apr 06 2008