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 A027990 #13 Jun 13 2015 00:49:08
%S A027990 1,2,7,21,65,194,573,1668,4809,13745,39011,110052,308857,862886,
%T A027990 2401195,6658509,18406373,50738918,139513269,382731240,1047768621,
%U A027990 2862908777,7808872007,21265115976,57822787825,157010774474,425795804623
%N A027990 Sum{T(n,k)*T(n,2n-k)}, 0<=k<=n, T given by A027926.
%H A027990 Reinhard Zumkeller, <a href="/A027990/b027990.txt">Table of n, a(n) for n = 0..1000</a>
%H A027990 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (5,-5,-5,5,-1).
%F A027990 (1/5)[n*F(2n+2) - n*F(2n-2) + 4*F(2n+1) + (-1)^n], F(n)=A000045(n). - _Ralf Stephan_, May 13 2004
%F A027990 G.f. ( 1-3*x+2*x^2+x^3 ) / ( (1+x)*(x^2-3*x+1)^2 ). - _R. J. Mathar_, May 22 2013
%o A027990 (Haskell)
%o A027990 a027990 n = a027990_list !! n
%o A027990 a027990_list = c [1] a000045_list where
%o A027990 c us (v:vs) = (sum $ zipWith (*) us vs) : c (v:us) vs
%o A027990 -- _Reinhard Zumkeller_, Nov 01 2013
%K A027990 nonn
%O A027990 0,2
%A A027990 _Clark Kimberling_