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 A086073 #7 Dec 15 2017 17:36:32 %S A086073 0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,1,0,0,0,0,1,1,1,1,0,1,0,0,0,0,0,0,1,0, %T A086073 0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,2,1,1,0,0,0,0,0,0,0,0,0,0,0,1,1,1,2,2, %U A086073 1,1,1,1,2,1,1,1,2,0,0,0,1,1,0,0,0,0,0,1,0,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,0 %N A086073 Number of 2's in decimal expansion of triangular number n(n+1)/2. %e A086073 tri(49)=1225, so a(49)=2 and tri(651)=212226, so a(651)=4. %t A086073 DigitCount[#,10,2]&/@Accumulate[Range[0,120]] (* _Harvey P. Dale_, Jan 02 2016 *) %Y A086073 Cf. 0's A086071, 1's A086072, 3's A086074, 4's A086075, 5's A086076, 6's A086077, 7's A086078, 8's A086079, 9's A086080. %K A086073 base,nonn %O A086073 0,50 %A A086073 _Jason Earls_, Jul 08 2003