  The criterion commonly used in directed acyclic graphs (dags) for testing
graphical independence is the well-known d-separation criterion. It allows us
to build graphical representations of dependency models (usually probabilistic
dependency models) in the form of belief networks, which make easy
interpretation and management of independence relationships possible, without
reference to numerical parameters (conditional probabilities). In this paper,
we study the following combinatorial problem: finding the minimum d-separating
set for two nodes in a dag. This set would represent the minimum information
(in the sense of minimum number of variables) necessary to prevent these two
nodes from influencing each other. The solution to this basic problem and some
of its extensions can be useful in several ways, as we shall see later. Our
solution is based on a two-step process: first, we reduce the original problem
to the simpler one of finding a minimum separating set in an undirected graph,
and second, we develop an algorithm for solving it.
