Square-Bracing Problem/Non-Crossing Rods
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Classic Problem
Let there be an unlimited supply of rigid rods of unit length which may be joined together at their ends with a hinged joint.
Let $4$ of these rods be used to construct a square.
How many more of these rods are needed to make the square rigid?
Rods are not allowed to cross.
Solution
This solution uses $23$ rods:
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Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $23$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $23$
- Weisstein, Eric W. "Braced Polygon." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/BracedPolygon.html