Directed graph

Description:

• A directed graph (or digraph) $(V,E)$ consists of a non-empty set of vertices $V$ and a set of directed edges (or arcs) $E$.
• When (𝑢, 𝑣) is an edge of the graph 𝐺 with directed edges, 𝑢 is said to be adjacent to 𝑣 and 𝑣 is said to be adjacent from 𝑢. The vertex 𝑢 is called the initial vertex of (𝑢, 𝑣), and 𝑣 is called the terminal or end vertex of (𝑢, 𝑣).
• The initial vertex and terminal vertex of a loop are the same.
• Types of directed graph:
  • Simple directed:
    • No loops and no multiple directed edges
  • Directed multigraph:
    • Have multiple directed edges from a vertex to a second (possibly the same) vertex
  • Multiplicity $m$:
    • When there are $m$ directed edge from $(u,v)$
  • Mixed graph:
    • When there are both undirected and directed edges

In-degree vs out-degree:

• In-degree of a vertex 𝑣, deg${}^{-}(v)$, is the number of edges with 𝑣 as their terminal vertex.
• The out-degree of 𝑣, denoted by deg${}^{+}(v)$, is the number of edges with 𝑣 as their initial vertex.
• tips to remember:
  • when branching out, have a chance to attack, you gain strength ⇒ out degree, $de{g}^{+}$
  • when being attacked inside, you lose strength ⇒ in degree, $de{g}^{-}$
• A loop at a vertex contributes 1 to both the in-degree and the out-degree of this vertex.
• ${\sum }_{v\in V}{\text{deg}}^{-}(v)={\sum }_{v\in V}{\text{deg}}^{+}(v)=|E|$