  Given an undirected graph G or hypergraph X model for a given set of
variables V, we introduce two marginalization operators for obtaining the
undirected graph GA or hypergraph HA associated with a given subset A c V such
that the marginal distribution of A factorizes according to GA or HA,
respectively. Finally, we illustrate the method by its application to some
practical examples. With them we show that hypergraph models allow defining a
finer factorization or performing a more precise conditional independence
analysis than undirected graph models.
