Common Graph Classes
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Questions and Answers

What is the definition of a path in a graph?

A graph G is called a path if its vertices can be (re)labeled u₁, u₂, ..., un so that its edges are u₁u₂, u₂u₃, ..., un-1un.

How is a path of order n denoted?

Pn

What is the definition of a complete graph?

A graph G is called complete if every pair of distinct vertices is adjacent.

How is a complete graph of order n denoted?

<p>K<sub>n</sub></p> Signup and view all the answers

For a complete graph of order n, what is the formula for its size (i.e., the number of edges)?

<p>$m = \binom{n}{2} = \frac{n(n-1)}{2}$</p> Signup and view all the answers

What is the definition of the complement of a graph G?

<p>The complement of a graph G, denoted by G, is the graph with vertex set V(G) such that for each pair of distinct vertices u, v of G, uv is an edge of G iff uv is not an edge of G.</p> Signup and view all the answers

If a graph G has order n and size m, what is the formula for the size of its complement, G?

<p>$m¯ = \binom{n}{2} - m = \frac{n(n-1)}{2} - m$</p> Signup and view all the answers

If G is a complete graph of order n, what is the description of Kn?

<p>K<sub>n</sub> has n vertices but zero edges and is called the empty graph of order n.</p> Signup and view all the answers

What is the definition of a bipartite graph?

<p>A graph G = (V, E) is bipartite if the vertex set V can be partitioned into two sets A and B (the bipartition or partite sets) such that no edge in E has both endpoints in the same set of the bipartition.</p> Signup and view all the answers

A nontrivial graph G is bipartite if and only if G contains no odd cycles.

<p>True (A)</p> Signup and view all the answers

What is the definition of the join of two graphs, G and H?

<p>The join G + H is the graph that consists of G ∪ H and all edges joining a vertex of G and a vertex of H.</p> Signup and view all the answers

What is the definition of the Cartesian product of two graphs, G and H?

<p>The Cartesian Product G × H is the graph with vertex set V(G × H) = V(G) × V(H). Two distinct vertices (u, v) and (x, y) are joined by an edge if either a) u = x and vy ∈ E(H), or b) v = y and ux ∈ E(G),</p> Signup and view all the answers

Explain how to draw the Cartesian product G × H.

<p>Replace each x ∈ E(G) by a copy of the graph H, call it H<sub>x</sub>. Then, join corresponding vertices of H<sub>x</sub> and H<sub>y</sub> if x and y are adjacent in G. Otherwise, we do not add any edges between H<sub>x</sub> and H<sub>y</sub>.</p> Signup and view all the answers

There is a graph of order n ≥ 2 whose vertices have all different degrees.

<p>False (B)</p> Signup and view all the answers

Flashcards

Path Graph (Pn)

A graph where vertices are labeled to create a linear connection.

Cycle Graph (Cn)

A graph where vertices form a closed loop with edges connecting in sequence.

Complete Graph (Kn)

A graph where every pair of distinct vertices is adjacent.

Graph Size (m)

The number of edges in a graph, calculated for complete graphs as m=n(n-1)/2.

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Graph Complement (Ḡ)

A graph that represents absence of edges; edges exist if they are not in G.

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Bipartite Graph

A graph whose vertices can be split into two sets with no edges within each set.

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Bipartite Condition

A nontrivial graph is bipartite if it contains no odd cycles.

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Bipartition Sets

Two sets (A and B) that divide the vertex set of a bipartite graph.

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Odd Cycle

A cycle containing an odd number of vertices, which prevents bipartitioning.

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Join of Graphs (G H)

Graph formed by combining G and H with edges connecting their vertices.

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Cartesian Product of Graphs

A graph where vertices from G and H create a new vertex set with specific edges.

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Graph Order (n)

The number of vertices in a graph.

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Empty Graph (K̄n)

A complete graph with zero edges, consisting only of vertices.

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Graph Degree

The degree of a vertex is the number of edges incident to it.

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Nontrivial Graph

A graph that has at least one edge (not just isolated vertices).

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Contradiction in Proof

A logical approach showing that assuming the opposite leads to an impossibility.

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Graph Edge

A line connecting two vertices in a graph.

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Graph Vertex

A fundamental unit by which graphs are formed, often represented as points.

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Connected Graph

A graph in which there is a path between every pair of vertices.

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Distinct Vertices

Different points in a graph with unique identifiers.

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Minimum Length Path

The shortest route that connects two points in a graph.

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Graph Size Formula for Complement

If G has order n and size m, Ḡ has order n and size n(n-1)/2 - m.

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Bipartition Definition

Dividing graph's vertices into two groups where edges only exist between groups.

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Graph Representation

Different ways to illustrate or depict the same graph structure.

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Partite Sets

The two separate sets in a bipartite graph arrangement (A and B).

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Proof by Contradiction

A method of proving a statement is true by showing that assuming it false leads to an incongruence.

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Eulerian Path

A path in a graph that visits every edge exactly once.

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Subgraph

A graph formed from a subset of a graph's vertices and edges.

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Study Notes

Common Graph Classes

  • A graph G is a path if its vertices can be relabeled (u₁, u₂, ..., uₙ) such that its edges are u₁u₂, u₂u₃, ..., uₙ₋₁uₙ.
  • A path of order n is denoted by Pₙ.
  • A graph G of order n ≥ 3 is a cycle if its vertices can be relabeled (u₁, u₂, ..., uₙ) such that its edges are u₁u₂, u₂u₃, ..., uₙ₋₁uₙ, uₙu₁.
  • A cycle of order n is denoted by Cₙ.
  • A graph G is complete if every pair of distinct vertices is adjacent.
  • A complete graph of order n is denoted by Kₙ.
  • The size (number of edges) of a complete graph of order n is given by m = n(n-1)/2.
  • The complement of a graph G (denoted G) is the graph with the same vertex set as G but with an edge between two vertices if and only if there is no edge between those vertices in G.
  • A graph G=(V,E) is bipartite if the vertex set V can be partitioned into two sets A and B (the partite sets) such that no edge in E has both endpoints in the same set.
  • A nontrivial graph G is bipartite if and only if G contains no odd cycles.

Definitions & Notation

  • The join of two graphs G and H (denoted G + H) is the graph consisting of the union of G and H, and all edges joining a vertex of G and a vertex of H.

Cartesian Product

  • If G and H are graphs, the Cartesian product G × H is the graph with vertex set V(G × H) = V(G) × V(H).
  • Two vertices (u, v) and (x, y) in V(G × H) are joined by an edge if either u = x and (v, y) is an edge in H, or v = y and (u, x) is an edge in G.

Problem

  • The problem involves drawing the join and Cartesian product of specific graphs (P₃ and K₃).

Theorem

  • There is no graph with order n ≥ 2 whose vertices have all different degrees.

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Description

Test your knowledge on the various common graph classes, including paths, cycles, complete graphs, and bipartite graphs. This quiz covers their definitions, notations, and properties essential for graph theory. Challenge yourself to see how well you understand these foundational concepts in mathematics.

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