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 A069712 #7 Jun 17 2017 10:26:10
%S A069712 1,3,6,15,28,55,66,91,105,120,153,171,210,231,276,300,351,378,435,465,
%T A069712 528,561,630,666,741,780,861,903,990,1128,1326,1830,2145,2415,3081,
%U A069712 5151,5995,6105,7140,8385,8646,9591,9870,10153,11026,11175,12246,12403
%N A069712 Triangular numbers with arithmetic mean of digits = integer (sum of digits = A multiple of the number of digits).
%H A069712 Harvey P. Dale, <a href="/A069712/b069712.txt">Table of n, a(n) for n = 0..1000</a>
%t A069712 Do[ If[ IntegerQ[ Apply[ Plus, IntegerDigits[n(n + 1)/2]] / Floor[ Log[10, n(n + 1)/2] + 1]], Print[n(n + 1)/2]], {n, 1, 10^3}]
%t A069712 Select[Accumulate[Range[200]],IntegerQ[Mean[IntegerDigits[#]]]&] (* _Harvey P. Dale_, Jun 17 2017 *)
%Y A069712 Cf. A069709, A069710, A069711.
%K A069712 base,nonn
%O A069712 0,2
%A A069712 _Amarnath Murthy_, Apr 08 2002
%E A069712 Edited and extended by _Robert G. Wilson v_, Apr 12 2002