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 A113978 #2 Mar 30 2012 18:59:12
%S A113978 1,11,101,1101,10001,111001,1000001,11010001,101000001,1100100001,
%T A113978 10000000001,111101000001,1000000000001,11000010000001,
%U A113978 101010000000001,1101000100000001,10000000000000001,111001001000000001
%N A113978 a(n)=Sum(d|n)(10^(n-d)).
%C A113978 A074854 in base 2. Stacking the elements gives the triangle A051731.
%F A113978 G.f.: Sum_{k>0} 10^(k-1)*x^k/(1-10^(k-1)*x^k).
%o A113978 (PARI) a(n)=if(n<1,0,10^n*polcoeff(sum(k=1,n,10/(10-x^k),x*O(x^n)),n))
%K A113978 easy,nonn
%O A113978 0,2
%A A113978 _Paul Barry_, Nov 11 2005