Abstract: Authores have studied the structure of R_G, which is the set of reversible edges of a dag(directed acyclic graph) G and present the sufficent and necessary condition to determine R_G. This result can help to search the equivalent classes of a dag and be used in the learning of Bayesian Network.

Key words: directed acyclic graph, reversible edge, compelled edge, clique, v-structure, system of parent and children

摘要： 讨论了有向非循环图(dag)G的可反向边集合R_G的结构，给出了判断R_G的充分必要条件。这一结果将有助于设计算法搜索G中的等价类，在用得分等价原则学习贝叶斯网络结构时，可以进行局部得分，从而减少所需的数据量，提高效率。

关键词: 有向非循环图, 可反向边, 不可反向边, clique, v-结构, 父子结构