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 A118401 #3 Mar 30 2012 18:36:57
%S A118401 1,0,1,2,0,1,-2,2,0,1,4,-2,2,0,1,-6,4,-2,2,0,1,8,-6,4,-2,2,0,1,-10,8,
%T A118401 -6,4,-2,2,0,1,12,-10,8,-6,4,-2,2,0,1,-14,12,-10,8,-6,4,-2,2,0,1,16,
%U A118401 -14,12,-10,8,-6,4,-2,2,0,1,-18,16,-14,12,-10,8,-6,4,-2,2,0,1,20,-18,16,-14,12,-10,8,-6,4,-2,2,0,1
%N A118401 Triangle, read by rows, equal to the matrix square of triangle A118400; also equals the matrix inverse of triangle A118407.
%C A118401 This triangle has an integer matrix square-root (A118400) if the main diagonal of the square-root is allowed to be signed. Even though the columns of this triangle are all the same, the columns of the matrix square-root A118400 are all different.
%F A118401 G.f.: A(x,y) = (1 + 2*x + 2*x^2)*(1+x^2)/(1+x)^2/(1-x*y). Column g.f.: (1 + 2*x + 2*x^2)*(1+x^2)/(1+x)^2.
%e A118401 Triangle begins:
%e A118401 1;
%e A118401 0, 1;
%e A118401 2, 0, 1;
%e A118401 -2, 2, 0, 1;
%e A118401 4,-2, 2, 0, 1;
%e A118401 -6, 4,-2, 2, 0, 1;
%e A118401 8,-6, 4,-2, 2, 0, 1;
%e A118401 -10, 8,-6, 4,-2, 2, 0, 1;
%e A118401 12,-10, 8,-6, 4,-2, 2, 0, 1;
%e A118401 -14, 12,-10, 8,-6, 4,-2, 2, 0, 1;
%e A118401 16,-14, 12,-10, 8,-6, 4,-2, 2, 0, 1; ...
%o A118401 (PARI) {T(n,k)=polcoeff(polcoeff((1+2*x+2*x^2)*(1+x^2)/(1+x)^2/(1-x*y+x*O(x^n)),n,x)+y*O(y^k),k,y)}
%Y A118401 Cf. A118400 (matrix square-root), A118402 (row sums), A118403 (unsigned row sums), A118407 (matrix inverse).
%K A118401 sign,tabl
%O A118401 0,4
%A A118401 _Paul D. Hanna_, Apr 27 2006