where 𝐷 𝑓 and 𝐷 𝐾𝑓 represent the diagonal matrices having the elements of vectors 𝑓 and 𝐾𝑓 on their main diagonals and 0 everywhere else, respectively. Since K is assumed to be invertible and diagonalizable, the matrix (𝐷 𝑓 𝐾 𝑇 -𝐷 𝐾𝑓 ) has a one-dimensional null-space. We can denote the basis of the null space by the vector 𝑣 0 (𝐾, 𝑓). Then 𝑓 * can be represented as with the following equation: