19

This number is a prime.

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Consider a square made of four rigid rods, hinged at the ends. How many rods of the same type must be added to make the square rigid in the plane? The answer is 19, which makes a total of 23 rods (also prime). [McCranie]

Submitted: 2005-03-08 18:36:47;   Last Modified: 2008-01-30 17:28:00.
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