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 A108555 #6 Nov 21 2012 08:54:05
%S A108555 1,2,5,12,35,116,411,1482,5333,19030,67265,235704,819863,2834600,
%T A108555 9752951,33428054,114228713,389419114,1325155261,4503031332,
%U A108555 15285291851,51842203804,175719341971,595316455842,2016131645245,6826076021310
%N A108555 Antidiagonal sums of square array A108553, in which row n equals the crystal ball sequence for D_n lattice.
%C A108555 Limit a(n+1)/a(n) ~ 3.3829757679..., real root of cubic (1+x+3*x^2-x^3).
%F A108555 Empirical G.f.: (x^3+x^2+x-1)*(2*x^5+5*x^4-4*x^3+10*x^2-6*x+1) / ((x-1)^2*(x^2+2*x-1)^2*(x^3+x^2+3*x-1)). [_Colin Barker_, Nov 21 2012]
%o A108555 (PARI) {a(n)=if(n<0,0,if(n==0,1,sum(k=0,n,sum(j=0,k,binomial(n-j,k-j)* (binomial(2*(n-k),2*j)-2*j*(n-k-j)*binomial(n-k,j)/if(n==k+1,1,(n-k-1)))))))}
%Y A108555 Cf. A108553, A108554.
%K A108555 nonn
%O A108555 0,2
%A A108555 _Paul D. Hanna_, Jun 10 2005