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Zonoid theory and Hilbert's fourth problem

✍ Scribed by Ralph Alexander


Publisher
Springer
Year
1988
Tongue
English
Weight
597 KB
Volume
28
Category
Article
ISSN
0046-5755

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✦ Synopsis


A finite vector sum of line segments is termed a zonotope. A zonoid is a Blaschke-Hausdorfflimit ofzonotopes. A projective metric d on a convex subset of projective space is shown to be of negative type if and only if the spheres in any tangent space are polar duals of zonoids. It follows that metric arclength can be represented by a Crofton formula with respect to a positive measure on the hyperplanes if and only if d is of negative type. These ideas allow a nice characterization of this cone of metrics.


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## Abstract We recall the characterisation of positive definite polynomial functions over a real closed ring due to Dickmann, and give a new proof of this result, based upon ideas of Abraham Robinson. In addition we isolate the class of convexly ordered valuation rings for which this characterisati