The problem of polygons with hidden vertices. (English) Zbl 1054.52005

Let \(P=[P_{1}P_{2}\dots P_{n}]\) be a polygonal path in \({\mathbb R}^3\) without self intersection. Ewald asked the question whether there exists such a polygon together with some point \(M\) not on the polygon with the property that for any \(i\) there exists some \(j\) such that the line segments \([M,P_{i}]\) and \([P_{j-1},P_{j}]\) intersect at a point in \((P_{j-1},P_{j})\). In [Beitr. Algebra Geom. 42, No. 2, 439–442 (2001; Zbl 0996.52006)] G. Ewald gave an example of such a configuration with \(n=14\). Ewald asked for the smallest number of vertices \(n_{\min}\) for which such a configuration exists, and proved \(8 \leq n_{\min} \leq 14\).
In the paper under review the author improves this inequality to \(11 \leq n_{\min} \leq 12\). Moreover, the author gives an example for \(n=12\).


52A37 Other problems of combinatorial convexity


Zbl 0996.52006
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