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 A138118 #14 Feb 18 2024 08:27:25
%S A138118 10,11100,11111000,11111110000,11111111100000,11111111111000000,
%T A138118 11111111111110000000,11111111111111100000000,
%U A138118 11111111111111111000000000,11111111111111111110000000000
%N A138118 Concatenation of 2n-1 digits 1 and n digits 0.
%C A138118 a(n) is also A147537(n) written in base 2. [From _Omar E. Pol_, Nov 08 2008]
%H A138118 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1010,-10000)
%F A138118 O.g.f.: 10*(1+100x)/[(-1+1000x)*(-1+10x)]. a(n)=A100706(n)*10^n = 10*a(n-1)+11*1000^n. - _R. J. Mathar_, Apr 03 2008
%e A138118 n .......... a(n)
%e A138118 1 ........... 10
%e A138118 2 ......... 11100
%e A138118 3 ....... 11111000
%e A138118 4 ..... 11111110000
%e A138118 5 ... 11111111100000
%t A138118 FromDigits/@Table[Join[PadRight[{},2n-1,1],PadRight[{},n,0]],{n,15}] (* _Harvey P. Dale_, Dec 09 2011 *)
%Y A138118 Cf. A138147, A138721.
%Y A138118 Cf. A147537.
%K A138118 easy,nonn,base
%O A138118 1,1
%A A138118 _Omar E. Pol_, Mar 29 2008