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 A138119 #21 Feb 27 2024 03:04:38
%S A138119 10,11000,11100000,11110000000,11111000000000,11111100000000000,
%T A138119 11111110000000000000,11111111000000000000000,
%U A138119 11111111100000000000000000,11111111110000000000000000000,11111111111000000000000000000000,11111111111100000000000000000000000
%N A138119 Concatenation of n digits 1 and 2*n-1 digits 0.
%C A138119 a(n) has 3*n-1 digits.
%C A138119 a(n) is also A147538(n) written in base 2. - _Omar E. Pol_, Nov 08 2008.
%H A138119 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1100,-100000).
%F A138119 From _Colin Barker_, Sep 16 2013: (Start)
%F A138119 a(n) = 1100*a(n-1) - 100000*a(n-2).
%F A138119 G.f.: 10*x / ((100*x-1)*(1000*x-1)). (End)
%e A138119 n ...... a(n)
%e A138119 1 ....... 10
%e A138119 2 ...... 11000
%e A138119 3 ..... 11100000
%e A138119 4 .... 11110000000
%e A138119 5 ... 11111000000000
%t A138119 LinearRecurrence[{1100, -100000}, {10, 11000}, 15] (* _Paolo Xausa_, Feb 06 2024 *)
%o A138119 (PARI) Vec(10*x/((100*x-1)*(1000*x-1)) + O(x^100)) \\ _Colin Barker_, Sep 16 2013
%Y A138119 Cf. A138118, A138145, A138146, A138147, A138719, A138720, A138721, A138722, A147538.
%K A138119 base,easy,nonn
%O A138119 1,1
%A A138119 _Omar E. Pol_, Apr 03 2008