A note on minimal art galleries
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We will consider some extensions of the polygonal art gallery problem. In a recent paper Morrison has shown the smallest (9 sides) example of an art gallery that cannot be observed by guards placed in every third corner. Author also mentioned two related problems, for which the minimal examples are not known. We will show that a polygonal fortress such that its exterior cannot be guarded by sentries placed in every second vertex has at least 12 sides. Also, we will show an example of three-dimensional polyhedron such that its inside cannot be covered by placing guard in every vertex which has both fewer vertices and faces than previously known.
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