Graph

Description:

• From Seven Bridges of Königsberg
• a graph $G=(V,E)$ consists of $V$, a non-empty set of vertices and $E$, a set of edges

Finite vs infinite graph:

• A graph is called finite if both $|V|$ and $|E|$ are finite; otherwise, it is infinite.

Multigraph:

• Graphs that may have multiple edges connecting the same vertices

Pseudographs

• include loops, and possibly multiple edges

illustration of simple, multi, pseudo graphs

• pic

Undirected graph

Directed graph

Types:

Туре	Edges	Multiple Edges Allowed?	Loops Allowed?
Simple graph	Undirected	No	No
Multigraph	Undirected	Yes	No
Pseudograph	Undirected	Yes	Yes
Simple directed graph	Directed	No	No
Directed multigraph	Directed	Yes	No
Mixed graph	Directed and undirected	Yes	Yes

• Simple graph:
  • Complete Graph
  • Cycle:
    • for $n\ge 3$, the edges forms a cycle
    • There are $n$ edges in a cycle graph with $n\ge 3$ vertices

Graph representation

Graph isomorphism

Path

Graph connectivity:

• An Undirected graph is connected if there exists a path between any two nodes $u,v\in V$definition
  • note that a graph containing a single node $v$ is considered connected via the length 0 path $(v))$
  • An undirected graph that is not connected is called disconnected
• There is a simple path between every pair of distinct vertices of a connected undirected graphtheorem
• A Directed graph is a strongly connected if there exists a path form any node $u$ to any node $v$ (so is from the node $v$ to node $u$)definition
• A Directed graph is a weakly connected if there is a path between every 2 vertices in the underlying Undirected graphdefinition
• A simple cycle of a particular length is a useful invariant that can be used to show that two graphs are not isomorphic.

Counting paths between vertices:

• Let 𝐺 be a graph with adjacency matrix 𝑨 with respect to the ordering $v_1,v_2,..,v_n$ of the vertices of the graph (with directed or undirected edges, with multiple edges and loops allowed).
• The number of different paths of length $r$ from $v_1$ to $v_j$ , where $r$ is a positive integer, equals the $(i,j)$th entry of $A^r$theorem
  • the matrix power to r, then we have the number of possible ways of going from one node to the other

Subgraph:

• Given a graph $G=(V,E)$, a subgraph of $G$ is simply a graph $G^{\prime }=(V^{\prime },E^{\prime })$ with $V^{\prime }\subseteq V$ and $E^{\prime }\subseteq (V^{\prime }\times V^{\prime })\cap E$.
  • We denote by $G^{\prime }\subseteq G$
• A connected component of graph 𝐺 = (𝑉, 𝐸) is a maximal connected subgraph; that is, it is a subgraph 𝐻 ⊆ 𝐺 that is connected, and any larger subgraph 𝐻’ (satisfying 𝐻’ ≠ 𝐻, 𝐻 ⊆ 𝐻’ ⊆ 𝐺) must be disconnected.
• We may similarly define a strongly connected component of a directed graph as a maximal strongly connected subgraph???

Bipartite Graph

Planar Graph

Graph Coloring

Graph for Machine Learning