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 A104880 #3 Mar 30 2012 18:59:06
%S A104880 1,1,2,3,5,8,14,26,53,117,280,721,1989,5844,18188,59652,205305,739041,
%T A104880 2775190,10846975,44039765,185391480,807776210,3637193486,16900721837,
%U A104880 80939650565,399061251260,2023408865997,10540656630133,56362743769012
%N A104880 Diagonal sums of a sum-of-powers triangle.
%C A104880 Diagonal sums of A104878.
%F A104880 a(0)=1, a(1)=1, a(n)=n+sum{k=2..floor(n/2), (k^(n-2k+1)-1)/(k-1)}, n>1.
%K A104880 easy,nonn
%O A104880 0,3
%A A104880 _Paul Barry_, Mar 28 2005