Question:

Let \(G\) be an undirected connected graph in which every edge has a positive integer weight. Suppose that every spanning tree in \(G\) has even weight. Which of the following statements is/are TRUE for every such graph \(G\)?

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To analyze spanning trees, focus on the parity of edge weights in cycles of the graph.
Updated On: Jan 23, 2025
  • All edges in \(G\) have even weight
  • All edges in \(G\) have even weight OR} all edges in \(G\) have odd weight
  • In each cycle \(C\) in \(G\), all edges in \(C\) have even weight
  • In each cycle \(C\) in \(G\), either all edges in \(C\) have even weight OR} all edges in \(C\) have odd weight
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The Correct Option is D

Solution and Explanation


If every spanning tree in \(G\) has an even weight, it implies a specific structure for the graph \(G\). Consider the properties:
Option (A): Incorrect. Not all edges in \(G\) need to have even weight; some edges can have odd weights as long as the total weight of spanning trees remains even.
Option (B): Incorrect. \(G\) may contain both even and odd weighted edges.
Option (C): Incorrect. It is not necessary for all edges in any cycle \(C\) to have even weight.
Option (D): Correct. For every cycle \(C\) in \(G\), the parity of the edges must be consistent (all even or all odd) to ensure that every spanning tree has an even total weight. Final Answer: \[ \boxed{\text{(D)}} \]
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