Proving "3f ≤ 2e" for Planar Graphs

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SUMMARY

The discussion centers on proving the inequality "3f ≤ 2e" for simple, connected planar graphs, where f represents the number of faces and e represents the number of edges. Participants highlight the concept of double counting, noting that each edge contributes to two faces. The proof involves recognizing that if you consider each edge as being counted twice, it leads to the conclusion that the relationship holds true. Understanding this double counting method is crucial for grasping the proof.

PREREQUISITES
  • Understanding of planar graph theory
  • Familiarity with the concepts of faces and edges in graph theory
  • Knowledge of double counting techniques in combinatorial proofs
  • Basic proficiency in mathematical proofs and inequalities
NEXT STEPS
  • Study the principles of planar graph theory and its properties
  • Learn about double counting techniques in combinatorial mathematics
  • Explore Euler's formula for planar graphs and its implications
  • Practice proving inequalities related to graph properties
USEFUL FOR

Students of graph theory, mathematicians focusing on combinatorial proofs, and educators teaching planar graph properties will benefit from this discussion.

kinof
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Homework Statement


Prove that for a simple, connected, planar graph, 3f ≤ 2e where f = faces and e = edges

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.
 
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hi kinof! :smile:
kinof said:
My professor went over this in class, and mentioned something about "double counting,"

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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