Flipping Edged in Triangulations

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There are two main important points which we will investigate. 

  • Number of Flippable EdgesThe most important question to ask naturally arises to how many edges can be flipped in given triangulation?  Is there a bound?  Using the applet can you think of a general formula for the number of edges which can be flipped?  

    We will show that there exists a tight bound on the number of edges which can be flipped in a given point set and that similar results follow for polygons.

    The question is trivially for small point sets and polygons but consider the following: 

How many edges can be flipped in this polygon?  

Is the question easy for the next triangulated polygon?  


          We note that the above figure was taken from the site of the University of Stuttgart.

  • Diameter of Triangulations: That the bound on the diameter of the graph of triangulations of a point set Pn that is sensitive to the number of convex layers of Pn.  

    What does the diameter of the graph of a triangulation show us?  If we define a graph of triangulations of a polygon Q to have a vertex set corresponding to the triangulations of Q and an edge set defined of the set of all adjacent vertices, with two vertices being adjacent if and only if the triangulation corresponding to one vertex can be obtained from the triangulation of the other corresponding endpoint by a single flip.  

Look at the diameter of the polygon on the left, AB.  If our polygon represents the graph of a triangulations of a polygon or point set.  What can we say about the diameter?  The diameter gives us the maximum distance from any two triangulations of a polygon.

Last Updated December 2, 2003 
By Christina Boucher