“…A clustering algorithm has to determine the appropriate number of clusters, as does for instance the Davies-Bouldin index [19], and to determine the measure that will permit to assign the elements into the said clusters [20]. PCA has been used as a clustering method among other techniques such as functional enrichment and expectation-maximization [21], kernel PCA [22], kernel PCA compared with the Davies-Bouldin index [20,23], Mercer kernelbased clustering [24], or spectral clustering [25]. Clustering has been applied to data mining [26], image processing [25], pattern recognition [27], self-organizing systems [28,29], and others.…”