Inappropriateness of the criterion of k-way normalized cuts for deciding the number of clusters

  • Authors:
  • Ayumu Nagai

  • Affiliations:
  • Department of Computer Science, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan

  • Venue:
  • Pattern Recognition Letters
  • Year:
  • 2007

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Abstract

Spectral clustering is a completely different algorithm from other existing clustering algorithms in that it relies on a linear algebraic approach including spectral decomposition. Normalized Cuts is a representative algorithm of spectral clustering. It incorporates a criterion for deciding the number k of clusters to partition. This paper shows that the criterion is not appropriate for deciding k. We showed this by proving that the optimal bipartition (that is, when k=2) becomes the optimal clustering. Namely, based on the criterion, the evaluation becomes better when k is small. We also show that the criterion is inappropriate for comparing approximate solutions with various k. Especially we prove that a bipartition which surpasses the best given approximate solution can be constructed from within the time complexity at most , where is the number of clusters contained in . Based on these two reasons, the Normalized Cuts Criterion is not appropriate for deciding k. An alternative criterion is necessary.