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Homework Help: Graph Theory Proof

  1. Jun 23, 2010 #1
    1. The problem statement, all variables and given/known data

    Prove that a graph is a tree if and only if it has no cycles and the insertion of any new edge always creates exactly one cycle.

    3. The attempt at a solution

    Assume that a graph G is connected and contains no vertices with a degree of zero.

    So would I get my proof by proving that the graph is a tree and it is connected and has n-1 edges which proves it has no cycles and then prove that adding a edge would create a cycle making the first statement false. Does this work?
  2. jcsd
  3. Jun 24, 2010 #2
    Two directions. In the first, you have a tree. Facts about trees: they are connected and acyclic. It is a fact that a tree has exactly n-1 edges, but depending on what you are allowed to assume, you may need to prove this. Starting from these facts, you must prove that adding any additional edge will create a cycle. In the second, you have a graph such that the insertion of any edge creates a cycle. You must prove that this graph is a tree, i.e. connected and acyclic.
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