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 A069699 #6 Dec 05 2013 19:55:17
%S A069699 0,1,3,6,10,15,21,28,36,45,55,66,78,91,171,276,378,1770,2775,5778,8778
%N A069699 Triangular numbers with internal digits 7.
%C A069699 For proof of finiteness see A069693.
%t A069699 Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {6}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
%Y A069699 Cf. A069675 to A069684, A069693 to A069700.
%K A069699 nonn,base,fini,full
%O A069699 1,3
%A A069699 _Amarnath Murthy_, Apr 06 2002
%E A069699 Corrected by _Sascha Kurz_, Jan 02 2003