Strongly chordal graph

In the mathematical area of graph theory, an undirected graph G is strongly chordal if it is a chordal graph and every cycle of even length (≥ 6) in G has an odd chord, i.e., an edge that connects two vertices that are an odd distance (>1) apart from each other in the cycle. Strongly chordal graphs have a forbidden subgraph characterization as the graphs that do not contain an induced cycle of length greater than three or an n-sun (n ≥ 3) as an induced subgraph. An n-sun is a chordal graph with 2n vertices, partitioned into two subsets U = {u1, u2,...} and W = {w1, w2,...}, such that each vertex wi in W has exactly two neighbors, ui and u(i + 1) mod n. An n-sun cannot be strongly chordal, because the cycle u1w1u2w2... has no odd chord. Strongly chordal graphs may also be characterized as the graphs having a strong perfect elimination ordering, an ordering of the vertices such that the neighbors of any vertex that come later in the ordering form a clique and such that, for each i < j < k < l, if the ith vertex in the ordering is adjacent to the kth and the lth vertices, and the jth and kth vertices are adjacent, then the jth and lth vertices must also be adjacent. A graph is strongly chordal if and only if every one of its induced subgraphs has a simple vertex, a vertex whose neighbors have neighborhoods that are linearly ordered by inclusion. Also, a graph is strongly chordal if and only if it is chordal and every cycle of length five or more has a 2-chord triangle, a triangle formed by two chords and an edge of the cycle. A graph is strongly chordal if and only if each of its induced subgraphs is a dually chordal graph. Strongly chordal graphs may also be characterized in terms of the number of complete subgraphs each edge participates in. Yet another characterization is given in. It is possible to determine whether a graph is strongly chordal in polynomial time, by repeatedly searching for and removing a simple vertex.