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Dec 8, 2019 at 2:05 history edited Joseph O'Rourke CC BY-SA 4.0
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Dec 7, 2019 at 22:21 comment added user44143 You could add: This path can be extended with a sawtooth pattern on each pair of rows below it, showing that there are appropriate paths on any larger triangles with even side lengths.
Dec 7, 2019 at 21:35 history edited Joseph O'Rourke CC BY-SA 4.0
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Dec 7, 2019 at 21:16 history edited Joseph O'Rourke CC BY-SA 4.0
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Dec 7, 2019 at 17:26 history edited Joseph O'Rourke CC BY-SA 4.0
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Dec 7, 2019 at 2:13 comment added Gerhard Paseman By affine invariance , the poster's suggested configuration is equivalent to the problem on a triangular grid. There one has that an extreme vertex must not be an endpoint, and with k=2 one has two cases to check, both of which isolate a vertex under the conditions. Gerhard "Let Symmetry Do The Work" Paseman, 2019.12.06.
Dec 7, 2019 at 1:28 comment added Joseph O'Rourke @GerhardPaseman: The OP clarified---Your interpretation is what he intended, mine was incorrect. Still my example shows there is no simple polygonal path through those $6$ points, turning at each.
Dec 7, 2019 at 0:43 comment added Joseph O'Rourke @GerhardPaseman: Ah, and I was assuming a "broken line" is a polygon. Your interpretation may be the correct one. Clearly there is confusion over this issue.
Dec 7, 2019 at 0:40 comment added Gerhard Paseman I interpret a broken line as a broken line segment, which has the (topological) shape of a path, not of a cycle. Gerhard "Who Sometimes Does Not Return" Paseman, 2019.12.06.
Dec 7, 2019 at 0:35 comment added Joseph O'Rourke @GerhardPaseman: What does it mean for a path to end at a vertex, when the path must be a closed cycle?
Dec 6, 2019 at 23:17 comment added Gerhard Paseman You can reduce this to whether a path ends at vertex 1 or not. When it doesn't, symmetry gives two cases that are easily checked. A slow divide and conquer algorithm can handle the general problem in a similar way. Gerhard "Knows No Fast Algorithm Yet" Paseman, 2019.12.06.
Dec 6, 2019 at 16:50 history undeleted Joseph O'Rourke
Dec 6, 2019 at 16:50 history edited Joseph O'Rourke CC BY-SA 4.0
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Dec 6, 2019 at 16:45 history deleted Joseph O'Rourke via Vote
Dec 6, 2019 at 16:37 history answered Joseph O'Rourke CC BY-SA 4.0