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 A102921 #4 Mar 30 2012 18:36:44
%S A102921 1,3,36,1036,56355,5045370,679409158,129195427716,33114233390505,
%T A102921 11038606786054201,4652371578279864792,2423023045813285312020,
%U A102921 1530233703568825263174101,1153422053136775523883308988
%N A102921 Column 0 of triangle A102920, which equals the matrix square of A102098.
%C A102921 Equals the bisection of A102917. Triangle A102098 shifts each column up 1 row under matrix cube.
%o A102921 (PARI) {a(n)=if(n==0,2,polcoeff(2-sum(k=0,n-1,a(k)*x^k*prod(j=2,k\2+2,1-j*x +x*O(x^n))),n))} /* at even-indices n=0,2,4,.. */
%Y A102921 Cf. A102920, A102098, A102917.
%K A102921 nonn
%O A102921 0,2
%A A102921 _Paul D. Hanna_, Jan 21 2005