Halkidi, M., Vazirgiannis, M. and Hennig, C. (2015) Method-independent
indices for cluster validation. In C. Hennig, M. Meila, F. Murtagh,
R. Rocci (eds.) Handbook of Cluster Analysis, CRC
Press/Taylor & Francis, Boca Raton.
Liu, Y, Li, Z., Xiong, H., Gao, X., Wu, J. and Wu, S. (2013)
Understanding and enhancement of internal clustering validation
measures. IEEE Transactions on Cybernetics 43, 982-994.