Brief View to Graph and Its Properties
A graph is a way of specifying relationships among a collection of items.A graph consists of a set of objects, called nodes (節點), or sometimes we call it vertx in mathematics, with certain pairs of these objects connected by links called edges (邊). Any finite graph can be geometrically represented by a drawing obtained in the following manner. First, draw a dot for each vertex. Then, for every edge $e \in E$ with two endpoints, draw a line between the dots representing the vertices of $V(e)$, and for every edge with only one vertex, draw a line from the dot representing that vertex to that dot itself.
For example, let $V = {v_1, v_2, v_3, v_4}$, and let $E = {e_1, e_2, e_3, e_4}$. Also, let $V(e_1) = {v_1 v_2 }$, $V(e_2 ) = {v_1 , v_3 }$, $V(e_3 ) = {v_2 , v_3 }$, and $V(e_4 ) = { v_3 , v_4 }$.
We say that two nodes are neighbors if they a reconnected by an edge. In many settings, however, we want to express asymmetric relationships ─ for example, that $A$ points to $B$ but not vice versa. (a) an undirected graph (無向圖), and (b) a directed graph (有向圖).
The main reason to categorize undirected and directed graph is to analyze how these nodes interact with each other on the graph, by checking their direction, which indicates how messages, diseases, or ideological content are transmitted. In mathematical graph theory, we can convert the graph into a matrix form. For example, given 3 agents, i.e. $N = {1,2,3}$, and if we draw a graph where a link between nodes 1 and 2, a link between nodes 2 and 3, but no link between nodes 1 and 3, the matrix form of this undirected or unweighted network can be represented as:
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