In the task of bilingual lexicon extraction, two document sets are similar to each other and should be clustered if the combination of the two can complement the content of each single set, which relates to the notion of homogeneity.
both the English part C e 1 of C 1 and the French part C f 1 of C 1 should be comparable to their counterparts (respectively the same for the French part C f 2 of C 2 and the English part C e 2 of C 2 ).