R. M. Assunção, M. C. Neves, G. Câmara and C. C. Freitas, Efficient regionalization techniques for socio-economic geographical units using minimum spanning trees. International Journal of Geographical Information Science Vol. 20, No. 7, August 2006, 797–811 http://www.dpi.inpe.br/gilberto/papers/assuncao_neves_camara_ijgis.pdf