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 A104609 #2 Mar 30 2012 17:31:12
%S A104609 1,3,0,2,2,2,1,0,1,0,1,1,3,1,2,1,2,2,4,3,3,3,4,4,3,0,0,0,4,1,2,0,1,2,
%T A104609 2,3,2,1,3,3,2,2,1,1,4,2,0,2,2,1,4,0,4,3,3,2,3,3,4,4,2,4,2,3,4,3,4,0,
%U A104609 1,1,0,1,0,1,1,0,1,1,1,1,1,2,1,1,1,2,4,2,1,1,2,2,3,1,1,3,1,4,1,1,4,1,4,1,2
%N A104609 Write the natural numbers in base 5 in a triangle with k digits in the k-th row, as shown below. Sequence gives the leading diagonal.
%C A104609 1
%C A104609 23
%C A104609 410
%C A104609 1112
%C A104609 13142
%C A104609 021222...
%t A104609 t = Flatten[IntegerDigits[Range[1500], 5]]; t[[Table[n(n + 1)/2, {n, 105}]]]
%Y A104609 Cf. A104606, A104607, A104608, A104610, A104611, A104612, A104613, A091425, A104614, A104615, A104616, A104617, A104618, A104619, A104620.
%K A104609 base,nonn
%O A104609 1,2
%A A104609 _Robert G. Wilson v_, Mar 16 2005