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 A104619 #2 Mar 30 2012 17:31:12 %S A104619 1,3,6,10,15,2,1,1,14,3,2,2,5,12,4,4,4,13,6,7,11,6,9,9,10,7,12,13,1,0, %T A104619 1,10,5,1,12,8,1,1,14,1,9,7,1,4,3,1,2,2,1,3,4,2,7,9,2,14,1,2,8,12,2,5, %U A104619 10,3,5,11,3,8,15,3,14,6,3,7,0,4,3,13,4,2,13,4,4,0,5,9,6,5,1,15,5,12,11,6 %N A104619 Write the natural numbers in base 16 in a triangle with k digits in the k-th row, as shown below. Sequence gives the leading diagonal. %C A104619 1 %C A104619 23 %C A104619 456 %C A104619 789A %C A104619 BCDEF %C A104619 101112... %t A104619 t = Flatten[ IntegerDigits[ Range[1900], 16]]; t[[Table[n(n + 1)/2, {n, 100}]]] %Y A104619 Cf. A104606, A104607, A104608, A104609, A104610, A104611, A104612, A104613, A091425, A104614, A104615, A104616, A104617, A104618, A104620. %K A104619 base,nonn %O A104619 1,2 %A A104619 _Robert G. Wilson v_, Mar 16 2005