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 A138720 #6 Oct 15 2013 17:40:47
%S A138720 101,111000111,111111000000111111,111111111100000000001111111111,
%T A138720 111111111111111000000000000000111111111111111,
%U A138720 111111111111111111111000000000000000000000111111111111111111111
%N A138720 Concatenation of k digits 1, k digits 0 and k digits 1, where k is the n-th positive triangular number.
%t A138720 Table[c=(n(n+1))/2;FromDigits[Join[PadRight[{},c,1],PadRight[{},c,0], PadRight[{},c,1]]],{n,10}] (* _Harvey P. Dale_, Oct 15 2013 *)
%Y A138720 Cf. A000217, 138711, A138179, A138721, A138722.
%K A138720 base,easy,nonn
%O A138720 1,1
%A A138720 _Omar E. Pol_, Mar 29 2008