for i = 2:2:n, for j = 2:2:n, t = (a(i-1,j-1)+a(i-1,j)+a(i,j-1)+a(i,j))/4; ...
   a(i-1,j-1) = t; a(i,j-1) = t; a(i-1,j) = t; a(i,j) = t;
