Fig. 2
From: Approximate search for known gene clusters in new genomes using PQ-trees
Exemplification of three different PQ-trees. \(T_2\) can be obtained from \(T_1\) by reversing the children of a Q-node (the left child of the root) and by reordering the children of a P-node (the right child of the root), so \(T_2 \equiv T_1\). The PQ-tree \(T_3\) can be obtained from \(T_1\) by deleting one leaf and permuting the children of the right child of the root, so \(T_1 \succeq _1 T_3\). Now, \(T_2 \succeq _1 T_3\) can be inferred because the \(\equiv\) is an equivalence relation. By the definition of frontier, \(F(T_1)=ABCDEFG\); \(F(T_2)=DCBAEGF\); \(F(T_3)=ABDFEG\)