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 A086072 #10 Dec 15 2017 17:36:31
%S A086072 0,1,0,0,1,1,1,0,0,0,0,0,0,1,1,1,1,1,2,1,1,1,0,0,0,0,1,0,0,0,0,0,0,1,
%T A086072 0,0,0,0,1,0,0,1,0,0,0,1,2,2,2,1,1,1,1,2,1,1,1,1,3,1,1,2,1,1,0,1,2,0,
%U A086072 0,1,0,0,0,1,0,0,0,0,1,1,0,1,0,0,0,0,1,0,1,0,0,1,0,1,0,0,0,0,1,0,0,2,0,0,0
%N A086072 Number of 1's in decimal expansion of triangular number n(n+1)/2.
%e A086072 tri(6)=21, so a(6)=1 and tri(1541)=1188111, so a(1541)=5.
%t A086072 Table[DigitCount[(n(n+1))/2,10,1],{n,0,110}] (* _Harvey P. Dale_, Apr 24 2011 *)
%t A086072 DigitCount[#,10,1]&/@Accumulate[Range[0,110]] (* _Harvey P. Dale_, Jun 25 2014 *)
%Y A086072 Cf. 0's A086071, 2's A086073, 3's A086074, 4's A086075, 5's A086076, 6's A086077, 7's A086078, 8's A086079, 9's A086080.
%K A086072 base,nonn
%O A086072 0,19
%A A086072 _Jason Earls_, Jul 08 2003