A salesperson needs to make a trip. She needs to start from a city, visit each of the remaining cities exactly once, and finally return to the same city from which she started.
Which one of the following options is then true?
A
Step-by-Step Solution
Insight: A Hamiltonian cycle requires every vertex to have a degree of exactly 2 within the cycle. Graph (ii) has three vertices of degree 2, which forces a contradiction at the central vertex. Graph (i) is a 4x4 grid, which is bipartite with equal partitions, allowing a valid cycle.
Exam route: For (ii), identify vertices with degree 2. Their incident edges must be in the cycle. This forces the central vertex to have degree 3 in the cycle, which is impossible. Thus, (ii) has no Hamiltonian cycle. For (i), a 4x4 grid has a known Hamiltonian cycle (e.g., a snake pattern that closes). Thus, (i) is possible, (ii) is not.
Learning route:
- Understand the goal: A trip visiting every city exactly once and returning to the start is a Hamiltonian cycle.
- Analyze Graph (ii): It has 5 vertices. The top-left, bottom, and top-right vertices each have exactly 2 connections (degree 2).
- Apply the Degree-Two Vertex Rule: In any Hamiltonian cycle, if a vertex has degree 2, both of its edges must be part of the cycle.
- Trace the forced edges in (ii): The three degree-2 vertices force 6 edges. However, these edges all converge on the central vertex, giving it a degree of 3 in the supposed cycle. A cycle can only have degree 2 for every vertex. This is a contradiction, so (ii) is impossible.
- Analyze Graph (i): It is a 4x4 grid graph. It is bipartite with 8 black and 8 white vertices. Since the partitions are equal, a Hamiltonian cycle is possible. We can explicitly construct one by tracing the perimeter and weaving through the center without repeating vertices.
- Conclusion: Possible for (i), not for (ii).