Definition: Undirected Graph, Vertices, Edges, Simple Graph 

An undirected graph or graph \(G\) is a triple \(G:=(V,E,\gamma)\) with the following properties:

1. \(V\) is a non-empty set of elements called vertices or nodes.
2. \(E\) is a set (non-empty or empty) of elements, called edges.
3. Vertices and edges are not the same objects, formally \(V\cap E=\emptyset\).
4. The map \(\gamma: E\mapsto \{X:~X\subseteq V,~1\le|X|\le 2\}\), assigns to every edge \(e\) its ends, denoted by \(\gamma(e)\).

Please note that the ends of an edge \(e\) can be two different vertices (this is the case if \(|\gamma(e)|=2\)) or one an the same vertex (this is the case if \(|\gamma(e)|=1\)).

If \(|\gamma(e)|=1\), i.e. if the ends of \(e\) are the same vertex, the edge \(e\) is called a loop.

Two edges \(e\) and \(e’\) are called parallel or multiple edges if \(\gamma(e)=\gamma(e’)\), (thus they have the same ends). If additionally \(|\gamma(e)|=1\), they are called multiple loops.

We call \(G\) a simple undirected graph (or simple graph), if it has no loops and no parallel edges. A simple graph is written \(G(V,E)\). An edge of simple graph \(e:=(x,y)\) is written as \(xy\) (or \(yx\)).

Example graph:

| | | | | created: 2014-03-12 09:24:20 | modified: 2017-01-11 13:20:12 | by: bookofproofs | references: [566], [570]

1.Definition: Complete Graph 

2.Definition: Cycle Graph 

3.Definition: Null Graph 

4.Definition: Complement Graph 

5.Definition: Size of a Graph 

6.Definition: Order of a Graph