Which of the following is impossible
a.

A graph with four vertices of degree 2, 1, 2, 3

b.

A graph with five vertices of degree 2, 3, 2, 3, 4

c.

A graph with four vertices of degree 2, 1, 3, 4

d.

A graph with five vertices of degree 2, 2, 2, 3, 4

To determine which of the graph configurations is impossible, we can apply the Handshaking Lemma, which states that the sum of the degrees of all vertices in a graph must be even (since each edge connects two vertices and therefore contributes 2 to the total degree count).

Let's analyze each option:

a. **Graph with four vertices of degree 2, 1, 2, 3**
   - Sum of degrees: \(2 + 1 + 2 + 3 = 8\)
   - The sum is even. This configuration is possible.

b. **Graph with five vertices of degree 2, 3, 2, 3, 4**
   - Sum of degrees: \(2 + 3 + 2 + 3 + 4 = 14\)
   - The sum is even. This configuration is possible.

c. **Graph with four vertices of degree 2, 1, 3, 4**
   - Sum of degrees: \(2 + 1 + 3 + 4 = 10\)
   - The sum is even. This configuration is theoretically possible based on the sum being even.

d. **Graph with five vertices of degree 2, 2, 2, 3, 4**
   - Sum of degrees: \(2 + 2 + 2 + 3 + 4 = 13\)
   - The sum is odd. This configuration is impossible as it violates the Handshaking Lemma.

Thus, option **d** is the impossible configuration, as it results in an odd sum of degrees, which contradicts the basic property that in any graph, the sum of the degrees of all vertices must be even.

Would you like a further explanation on any of these points or help with another question?

Related questions you might consider:
1. How would you verify the possibility of constructing a graph for option c?
2. Can you provide a drawing of a graph described in option a or b?
3. What is the Handshaking Lemma and why is it important in graph theory?
4. Could you explain how to determine the number of edges in a graph based on the degrees of its vertices?
5. How do the degrees of vertices affect the connectivity of a graph?
6. What is an Euler circuit, and which of these graphs could potentially have one?
7. Discuss the implications of having high-degree vertices in terms of graph structure and traversal efficiency.
8. Explain the concept of simple graphs versus multigraphs and how they relate to these examples.

Tip: Always check the sum of the degrees and the number of vertices when assessing the possibility of graph configurations. This can help avoid common mistakes in conceptualizing graph structures.

Explore which graph configurations are impossible based on the degrees of vertices. Understand the application of the Handshaking Lemma in graph theory to solve these types of problems effectively. Suitable for advanced high school students and anyone interested in graph theory concepts.

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