function Dk = getdk(k)
%input: the column number of points, i.e. the k
%ouput: the Dk
Dk_1 = zeros(1, length(k));
Dk_2 = zeros(1, length(k));
Dk = zeros(1, length(k));

for ii = 1:length(k)   

    Dk_1(ii) = (0.6^2 + 0.2^2) * 512 * 0.3 + 2 * k(ii) * (0.6 - 0.3) * 0.2^2;
    Dk_2(ii) = (0.6^2 + 0.2^2) * 512 - 2 * k(ii) * (0.6 - 0.3) * 0.6;
    Dk(ii) = Dk_1(ii)/Dk_2(ii);
end;