Directed and undirected graphs can represent different distributions.
Ex: \(1 \rightarrow 2 \rightarrow 3 \Rightarrow 1 - 2 - 3\). However, so too does \(1 \leftarrow 2 \leftarrow 3 \Rightarrow 1 - 2 - 3\) as does \(1 \leftarrow 2 \rightarrow 3 \Rightarrow 1 - 2 - 3\). So 3 directed graphs can be represented by one undirected graph.
Ex: \(1 \rightarrow 2 \leftarrow 3\) has no equivalent undirected graph with these and only these conditional independencies.
Ex: \(1 - 2 - 3 - 4 - 1\) has no equivalent directed graph. Because the undirected graph has a loop, at some point, 2 edges will collide. Suppose the collision happens at \(3 \rightarrow 4 \leftarrow 1\). In the undirected graph, we have \(1 \perp 3 \lvert 2, 4\). But in the directed graph, we have \(1 \not\perp \lvert 2, 4\).
Efficient Representation: Suppose \(G_1, G_2\) are two graphs with corresponding families \(P(G_1), P(G_2)\) and with sets of CIs given by \(I(G_1), I(G_2)\). If a distribution \(p\) is representable by both graphs i.e. \(p \in P(G_1), p \in P(G_2)\), we say \(G_1\) is a more efficient representation of \(p\) if \(I(G_1) \subset I(G_2)\).
Independence Map: A DAG or UG is called an independence map (or I-map) for distribution \(p\) if set of CIs expressed by graph \(I(G) \subset I(p)\), where \(I(p)\) is the set of CIs expressed by the distribution.
The implication of being an I-map is that \(p\) is one of the distributions in the set of families represented by the graph.
Perfect Map: A graph \(G\) is a perfect map for distribution \(p\) if \(I(G) = I(p)\).
Minimal I-Map: A graph \(G\) is a minimal I-map if removing any edge would cause it to no longer be an I-map for distribution \(p\).
Note: A minimal I-map for a distribution \(p\) may not be unique. It depends on the choice of topological ordering and which CIes (if any) are dropped.
Moralization: An undirected graph \(G' = (V, E')\) is a moralization of a DAG \(G=(V, E)\) if \((i, j) \in E \Rightarrow (i, j) \in E'\) and if \(i \rightarrow k \leftarrow j\), we connect \(i\) to \(j\) (either direction fine). This connection is called a “moralization” after “marrying” two unmarried nodes.
Claim: DAG \(G\) has undirected perfect map \(\Leftrightarrow\) moralization of G does not add any edges.
Chordal: An undirected graph (UG) is chordal if every loop of size \(\geq 4\) has a chord (an edge connecting 2 non-consecutive nodes in loop)
Claim: Undirected graph G has a directed P-map \(\Leftrightarrow\) G is chordal