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I want to find some characteristics for a set of points $S$ which contains $n$ points and has some Voronoi Diagram $V(S)$. This diagram should have exactly $2n-5$ vertices.

I tried to use the Euler formula for planar graphs which says $v-e+f = 2$:

$\Rightarrow 2n -5 -e + n = 2 \Rightarrow e = 3n-7$ - but what can I now do with the information that $V(S)$ has $3n-7$ vertices?

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You made a small mistake by thinking that the Voronoi diagram is a planar graph. Remember that it has those rays that go to infinity. you can make a planar graph out of the Voronoi diagram by adding a new vertex and making all rays incident to this "apex". Plugging this into Euler's formula yields

$$ 2n-4 - e + n = 2 \quad \Rightarrow \quad e= 3n - 6.$$

A planar graph has $3n-6$ edges if it is a triangulation (this can als be obtained via Euler's formula – just set $3f=2e$). As a consequence the dual graph to your Voronoi diagram (the Delaunay tessellation) is a triangulation. Or phrased differently, every vertex of your Voronoi diagram has degree~3 and there are three rays.

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  • $\begingroup$ Thank you for this. I want exactly $3n-6$ edges...the definintion of a triangulation says there are $<= 3n-6$ edges. is this a problem? $\endgroup$ – 今天春天 May 22 '16 at 7:56
  • $\begingroup$ Its exactly $3n-6$ edges if the outer face is a triangle (which corresponds to the 3 rays in the Voronoi diagram). $\endgroup$ – A.Schulz May 23 '16 at 6:05
  • $\begingroup$ What if I want to have a Voronoi Diagram with exactly one vertex? $\endgroup$ – 今天春天 May 23 '16 at 18:42

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