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 A105621 #3 Mar 30 2012 18:36:45
%S A105621 1,-3,-7,-39,-315,-3243,-40167,-579159,-9514395,-175345083,
%T A105621 -3582404487,-80368306119,-1964266364475,-51955106653323,
%U A105621 -1478719724319207,-45062796853058679,-1463985729352297755,-50509695778598958363,-1844377821680825976327
%N A105621 Column 2 of triangle A105620.
%C A105621 A105620 is the matrix inverse square-root of triangle A105615.
%o A105621 (PARI) {a(n)=local(R,M=matrix(n+3,n+3,m,j,if(m>=j,if(m==j,1,if(m==j+1,-2*j, polcoeff(1/sum(i=0,m-j,(2*i)!/i!/2^i*x^i)+O(x^m),m-j)))))); R=(M+M^0)/2;for(i=1,floor(2*log(n+2)),R=(R+M*R^(-1))/2); return(if(n<0,0,R[n+3,3]))}
%Y A105621 Cf. A105615, A105620.
%K A105621 sign
%O A105621 0,2
%A A105621 _Paul D. Hanna_, Apr 16 2005