9.2 Practice Question
Duration: 2 min
This video lesson is available to enrolled students.
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This educational video segment focuses on solving graph theory practice problems using the Handshaking Lemma. The instructor demonstrates how to determine vertex constraints based on edge counts and degree limits. Key concepts include the relationship where the sum of all vertex degrees equals twice the number of edges (2|E|). The session covers two primary problem types: finding maximum vertices given a minimum degree constraint and finding minimum vertices given a maximum degree constraint. The instructor applies inequalities to bound the number of vertices, ensuring results are rounded appropriately for integer solutions.
Chapters
0:00 – 2:00 00:00-02:00
The instructor begins by addressing a problem asking for the maximum number of vertices in a simple graph with 35 edges where each vertex has a degree of at least 3. The on-screen text displays the question: 'Q Maximum no of vertex in a simple graph with 35 edges and degree of each vertex is at least 3 is ____?'. The instructor applies the Handshaking Lemma, calculating the sum of degrees as 35 * 2 = 70. To find the upper bound for vertices, they divide this sum by the minimum degree (3), resulting in 70/3. The calculation is shown as '20 / 3' on screen, which the instructor rounds down to find the maximum possible integer vertices. The final answer is circled as 23, though the arithmetic shown suggests a discrepancy or specific rounding logic used in class.
2:00 – 2:25 02:00-02:25
The video transitions to a second problem asking for the minimum number of vertices possible in a simple graph with 41 edges where each vertex has a degree of at most 5. The text on screen reads: 'Q Minimum number of vertices possible in a simple graph if 41 edges and degree of each vertex is at most 5?'. The instructor calculates the total degree sum as 41 * 2 = 82. They set up an inequality where the maximum degree (5) times the number of vertices (|V|) must be at least 82, written as '5 x |V| >= 82'. Solving for |V| gives approximately 16.4, which is displayed as '< 16.4'. Since vertices must be whole numbers, the instructor rounds up to conclude the minimum number of vertices is 17.
The lecture effectively demonstrates the practical application of the Handshaking Lemma for bounding graph parameters. The instructor consistently uses the formula sum(degrees) = 2 * edges to establish a total degree budget. For maximum vertex counts, the strategy involves dividing this budget by the minimum allowed degree per vertex and rounding down. Conversely, for minimum vertex counts, the instructor divides the budget by the maximum allowed degree and rounds up. This approach ensures that the resulting integer vertex count satisfies the given edge and degree constraints simultaneously.