It's a prime number, of course. It is also the hypotenuse of the smallest Pythagorean triangle. Phillips, R. 1994. Wells, D. 1997. The Penguin Dictionary of Curious and Interesting Numbers, rev.
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He is the author of The Mathematical Tourist, Islands of Truth, Newton's Clock, Fatal Defect, and The Jungles of Randomness. For children ages 10 and up.