Josep Domingo-Ferrer Francesc Sebé Optimal Multivariate 2-Microaggregation for Microdata Protection: A 2-Approximation. 129-138 2006 conf/psd/2006 Privacy in Statistical Databases http://dx.doi.org/10.1007/11930242_12 db/conf/psd/psd2006.html#Domingo-FerrerS06