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 A027909 #12 May 13 2013 11:47:13 %S A027909 1,4,21,112,615,3432,19383,110448,633726,3656360,21191555,123286440, %T A027909 719539015,4210967880,24702429825,145210795200,855172338570, %U A027909 5044470461352,29799593861974,176268499363840,1043889366927771,6188748520285584,36726461258947569,218146172715259488,1296812014083995850 %N A027909 T(2n,n-1), T given by A027907. %F A027909 G.f.: -g*(g^2+g+1)/(3*g^2+g-1) where g = x times the g.f. of A143927. - _Mark van Hoeij_, Nov 16 2011 %p A027909 seq(add(binomial(j,2*j-3*n-4)*binomial(2*n+2,j),j=0...2*n+2),n=0..25); # _Mark van Hoeij_, May 12 2013 %o A027909 (PARI) a(n)=sum(j=0,2*n+2,binomial(j, 2*j-3*n-4)*binomial(2*n+2, j)); \\ _Joerg Arndt_, May 13 2013~ %K A027909 nonn %O A027909 0,2 %A A027909 _Clark Kimberling_ %E A027909 Corrected offset, more terms, _Joerg Arndt_, May 13 2013