In eq.( ), 𝒚 ! (𝑗) and 𝒙 ! (𝑗) represents the j th data point of the class 𝒚 ! (or the cluster represented by the class 𝒚 ! ) and the j th data point of the class 𝒙 ! (which is the class prior to the application of the LDA). Also in eq.( ),