This is a very elementary question: I am not sure I understand the definition of a DAG correctly; I read on Wikipedia that there are three possible DAGs with 2 vertices. I can see A->B and B->A, but what is the third one?
I tried to look this up for quite a while, but could not find it. It seems that all the vertices of a graph need to be connected to at least one other vertex, so that rules out A and B without an edge between them. But again, what is the third one then?