<font color="#ff8000">罗宾逊 Robinson </font>(1973)研究了有向无环图的<font color="#ff8000"> 图计数 Graph enumeration </font>问题。<ref name="enum">{{citation|first=R. W.|last=Robinson|contribution=Counting labeled acyclic digraphs|pages=239–273|editor-first=F.|editor-last=Harary|editor-link=Frank Harary|title=New Directions in the Theory of Graphs|publisher=Academic Press|year=1973}}. See also {{citation
+
罗宾逊 Robinson (1973)研究了有向无环图的<font color="#ff8000"> 图计数 Graph enumeration </font>问题。<ref name="enum">{{citation|first=R. W.|last=Robinson|contribution=Counting labeled acyclic digraphs|pages=239–273|editor-first=F.|editor-last=Harary|editor-link=Frank Harary|title=New Directions in the Theory of Graphs|publisher=Academic Press|year=1973}}. See also {{citation
|last1 = Harary | first1 = Frank | author1-link = Frank Harary | first2 = Edgar M. | last2 = Palmer | year = 1973| title = Graphical Enumeration | publisher = [[Academic Press]] | isbn = 978-0-12-324245-7 | page=19}}.</ref>
|last1 = Harary | first1 = Frank | author1-link = Frank Harary | first2 = Edgar M. | last2 = Palmer | year = 1973| title = Graphical Enumeration | publisher = [[Academic Press]] | isbn = 978-0-12-324245-7 | page=19}}.</ref>