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Homework Help: Graph planarity

  1. May 13, 2012 #1
    1. The problem statement, all variables and given/known data
    Prove that for a simple, connected, planar graph, 3f ≤ 2e where f = faces and e = edges

    3. The attempt at a solution
    My professor went over this in class, and mentioned something about "double counting," which I do not really understand. I tried saying that it takes 2 counts to define an edge and 3 to define a face, but from here I am not sure what to do as I am unfamiliar with double-counting proofs.

    Thanks for the help.
  2. jcsd
  3. May 13, 2012 #2


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    hi kinof! :smile:
    every edge has exactly two faces, so slice every edge in two, lengthwise …

    then you have double the number of edges, and every edge has only one face :wink:
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