In graph theory, would a graph with edges $(u, v)$ and $(v, u)$ connecting vertices $v$ and $u$ be considered a directed simple graph? For example: would this graph be considered a simple directed graph?
Thanks for your help.
Best Answer
Yes, that is a simple directed graph (it has neither loops nor multiple arrows with the same source and target). However it is not a directed acyclic graph, because $u,v$ (or $1,3$ in your picture) form a directed cycle. See here for a description of many directed graph types.
Yes, it is called the size of a complete graph on $n$ vertices. You can also write it as ${n\choose 2}$ since you have $n$ points and any $2$ of them define an edge.
Read a bit more carefully the definition that your book gives: "A directed graph may have multiple directed edges from a vertex to a second (possibly the same) vertex are called as directed multigraphs."
The key thing to notice here is that the multiple directed edges have the same origin and destination. Thus, in your first graph there is only one directed edge from vertex $c$ to vertex $d$ (and also only one directed edge from $d$ to $c$). So this graph is just a directed graph. On the other hand, in the second graph, there are two edges from $e$ to $d$, and two edges from $b$ to $c$. So this graph is a directed multigraph.
Best Answer
Yes, that is a simple directed graph (it has neither loops nor multiple arrows with the same source and target). However it is not a directed acyclic graph, because $u,v$ (or $1,3$ in your picture) form a directed cycle. See here for a description of many directed graph types.