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Ptolemy metric space. Help

  1. Mar 7, 2009 #1
    Ptolemy metric space. Help!!

    The problem is :
    "Let x,y,z,t belongs to R^n where d(x,y)=||x-y||.
    Show that(Ptolemy's inequality):

    I have found this related to the topic paper but I cannot show that the Euclidean space R^n is Ptolemy.
    The paper in the second page "2.Preliminaries" says that "To show that the Euclidean space R^n is Ptolemy, consider again four points x,y,z,w. Applying a suitable Mobius transformation we can assume that z is a midpoint of y and w, i.e. |yz|=|zw|=1/2 |yw|. For this configuration...."
    But how can we extract from the above paragraph that the Euclidean space R^n is Ptolemy and which is the "suitable Mobius transformation"??

    Thanks anyone in advance.
  2. jcsd
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