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 A117376 #2 Mar 30 2012 18:59:14
%S A117376 1,3,13,55,240,1057,4688,20879,93250,417272,1869725,8386137,37640961,
%T A117376 169042436,759470361,3413223919,15343588991,68988110756,310233741889,
%U A117376 1395268187372,6275807975664,28230370862051,126996552221811
%N A117376 Expansion of 1/((1-3x*c(x))(1-x^2*c(x))), c(x) the g.f. of A000108.
%C A117376 Diagonal sums of number triangle A117375.
%F A117376 a(n)=sum{k=0..floor(n/2), [k<=n-k]*sum{j=0..n-k, (j/(n-k-j))*C(2n-k-j,n-k-j)[k<=j]*3^(j-k)}}
%K A117376 easy,nonn
%O A117376 0,2
%A A117376 _Paul Barry_, Mar 10 2006