# Graphs: Introduction and Terminology

1. Introduction

A graph (G) is a set of vertices (V) and set of edges (E).
The set V is a finite, nonempty set of vertices. The set E is a set of pair of
vertices representing edges.

G= (V, E)
V (G) = Vertices of graph G
E (G) = Edges of graph G

An example of graph is shown below:

The set representation for graph is:

V (G) = {A, B, C, D, E, F}
Applications
Graphs have many important applications as shown below:
– Analysis of electrical circuit
– Finding shortest routes
Project planning
– To represent highway structures
– Communication lines
– Railway lines
2. Undirected Graph
A graph containing unordered pair of vertices is called an
undirected graph. In an undirected graph, pair of vertices (A, B) and (B, A) represent
the same edge.

An example of undirected graph is show below:

V = {1, 2, 3, 4, 5}
E = {(1, 2), (1, 3), (1, 5), (2, 3), (2, 4), (3, 4), (4, 5)}
3. Directed Graph
A graph contain ordered pair of vertices is called a
directed graph. If an edge is represented using a pair of vertices (A, B) then
the edge is said to be directed from A to B.

In a directed graph, the pairs (A, B) and (B, A) represent two
different edges of a graph. Example of directed graph is shown below.

V = {1, 2, 3, 4, 5}
E = {(1, 3), (1, 5), (2, 1), (2, 4), (3, 4), (4, 5)}
4. Complete Graph
An undirected graph, in which every vertex has an edge to
all other vertices, is called a complete graph. A complete graph with N
vertices has (N (N-1))/2 edges.

5. Weighted Graph
A weighted graph is a graph in which edges are assigned some
value. For example, an edge may represent a highway link between two cities.
The weight will denote the distance between connected cities using highway.
Weight of an edge is also called its cost.

Two vertices A and B are said to be adjacent if there is an
edge between A and B.
7. Path
A path V0 to Vn is a sequence of
vertices V0, V1, V2 . . . Vn-1, Vn.
V2 and Vn-1 is adjacent to Vn. The length of a
path is the number id edges in the path. A path with n vertices has a length of
n-1. A path is simple if all vertices on the path, except possibly the first
and last, are distinct.
8. Cycle
A cycle is a simple path that begins and ends at the same
vertex. An example is shown below:

A B D A is a cycle of length 3
A B C D A is a cycle of length 4
9. Connected Graph
A graph is said to be connected if there exists a path
between every pair of vertices Vi and Vj.

10. Subgraph
A subgraph of G is a graph G1 such that V (G1)
is a subset of V (G) and E (G1) is a subset of E (G).

11. Component
A component H of an undirected graph is maximal connected
subgraph. The graph in below figure has three components H1, H2
and H3.

12. Degree of a Vertex
The total number of edges linked to a vertex is called its
degree.
Indegree: The
indegree of a vertex is the total number of edges coming to that node.
Outdgree: The
outdgree of a node is the total number of edged going out from that node.
Source: A vertex,
which has only outgoing edges and no incoming edges, is called a source.
Sink: A vertex
having only incoming edges and no outgoing edges is called sink.
Pendant: When
indegree of a vertex is one and outdegree is zero then such a vertex is called
pendant vertex.
Isolated: When
the degree of a vertex is zero, it is an isolated vertex.

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13. Self Edges or Self
Loops
An edge of the form (V, V) is known as self edge or self
loop.

14. Multigraph
A graph with multiple occurrences of the same edge is known
as a miltigraph.

15. Tree
A tree is a connected graph without any cycle. Tree is a
special case of a graph. When a tree is treated as a graph, it need not have a special
vertex called the root.

16. Spanning Tree
A spanning tree of a graph G = (V, E) is a connected
subgraph of G having all vertices of G and no cycles in it. If the graph G is
not connected then there is no spanning tee of G. A graph may have multiple
spanning trees.

17. Minimal Spanning
Tree
The cost of a graph is the sum of the costs of the edges in
the weighted graph. A spanning tree of a group G = (V, E) is called a minimal
cost spanning tree or simply the minimal spanning tree of G if its cost is
minimum.

G → A sample weighted graph
T1 → A spanning tree of G with cost 5 + 9 = 14
T2 → A spanning tree of G with cost 10 + 9 = 19
T3 → A spanning tree of G with cost 5 + 10 = 15
Therefore T1 with cost 14 is the minimal cost
spanning tree of the graph G.