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 A047090 #5 Mar 30 2012 18:56:53
%S A047090 1,2,3,6,12,21,42,84,155,310,620,1176,2352,4704,9071,18142,36284,
%T A047090 70717,141434,282868,555268,1110536,2221072,4381656,8763312,17526624,
%U A047090 34697879,69395758,138791516,275467455,550934910,1101869820,2191008499
%N A047090 Sum{T(i,n-i): i=0,1,...,n}, array T as in A047089.
%K A047090 nonn
%O A047090 0,2
%A A047090 _Clark Kimberling_