A common definition of directed graph is as follows:
A directed graph G =(N,E) consists of a nonempty set of vertices (or nodes) N, a set of directed edges E and a function which assigns each edge an ordered pair of vertices (a tail and a head).
However, the above definition does not consider the case of edges having only one vertex as it frequently occurs when a graph represents an open compartment system.
An alternative definition that can include open compartment graphs may be:
A directed graph G =(N,E) consists of a nonempty set of vertices (or nodes) N, a set of directed edges E, two subsets E1 and E2 of E such that their union is E, a function f which assigns each edge a vertex (tail) and a function g which assigns each edge a vertex (head)
When E1=E2=E the two definitions are equivalent.
What is your opinion about that? Do you think is it correct?
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