Saturday, 7 June 2008

mg.metric geometry - Intrinsic metric with no geodesics

There is a very simple example of an intrinsic, complete metric space that is not geodesic (read in Ballmann's "Lectures on Spaces of Nonpositive Curvature": it is the graph on two vertices x,y, linked by edges en of length 1+1/n.



Of course it does not answer your question, but it may be possible to improve this example to one that does. Call X1 the graph described above, and define Xn+1 from Xn
as follows: Xn has a vertex x for each vertex x of Xn, plus a vertex ve for each edge e of Xn. For each edge e=(xy) of Xn we define edges fne and gne of Xn+1: fne connects x to ve and has length (1+1/n) times the original length of e, and gne does the same
but replacing x by y.



Now it should be possible to construct the desired example by a limiting process. For example, take all vertices along the construction: the distance between any two of this points is constant as long it is defined, so we get a metric space. Its completion might be what you want (but I a not so sure of that after witting these lines).

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