Let \( G \) have 5 vertices and 3 edges. - Parker Core Knowledge

Understanding the Context

What Defines a Graph with 5 Vertices and 3 Edges?

In graph theory, a graph consists of vertices (or nodes) connected by edges. A graph with 5 vertices and 3 edges means we're working with a small network having only three connections among five points.

This sparsely connected structure fits many real-world models—like simple social connections, basic circuit diagrams, or minimal physicaical risks in network systems.

Key Insights

How Many Non-Isomorphic Graphs Exist?

Not all graphs with 5 vertices and 3 edges are the same. To count distinct configurations, graph theorists classify them by isomorphism—that is, shape or layout differences that cannot be transformed into each other by relabeling nodes.

For 5 vertices and 3 edges, there are exactly two non-isomorphic graphs:

1. A Tree
   This is the simplest acyclic graph—a connected graph with no cycles. It consists of a spine with three edges and two isolated vertices (pendant vertices). Visualize a central vertex connected to two leaf vertices, and a third leaf attached to one of those—forming a “Y” shape with two terminals.

Example layout:
A
|
B — C
|
D — E

Final Thoughts

This tree has:

• 5 vertices: A, B, C, D, E
  
• 3 edges: AB, BC, CD, CE (though E has only one edge to maintain only 3 total)

Note: A connected 5-vertex graph must have at least 4 edges to be a tree (n − 1 edges). Therefore, 3 edges ⇒ disconnected. In fact, the tree with 5 vertices and 3 edges consists of a main branch with two leaves and two extra terminals attached individually.

2. Two Separate Trees
   Alternatively, the graph can consist of two disconnected trees: for instance, a tree with 2 vertices (a single edge) and another with 3 vertices (a path of two edges), totaling 2 + 3 = 5 vertices and 1 + 2 = 3 edges.

Example:

• Tree 1: A–B (edge 1)
  
• Tree 2: C–D–E (edges 2 and 3)
  Total edges: 3, vertices: 5.

Key Graph Theory Concepts to Explore