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 A027139 #4 Mar 30 2012 18:56:14 %S A027139 2,7,23,72,218,648,1907,5581,16290,47502,138521,404183,1180402, %T A027139 3450899,10099605,29589780,86781374,254761882,748580729,2201465799, %U A027139 6479301022,19083705249,56246157177,165881453558,489506759446 %N A027139 T(n,n+1) + T(n,n+2) + ... + T(n,2n), T given by A027113. %K A027139 nonn %O A027139 0,1 %A A027139 _Clark Kimberling_