## Abstract

A positive whole number *N* is called a *prime* number if the only whole numbers which divide exactly into it are 1 and *N* itself. For example, of the first twenty numbers, 2, 3,5,7, 11, 13, 17, 19 are primes whereas 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20 are not. (The number 1 is conventionally excluded from the category of primes.) Except for the number 2, all primes are odd — a fact which makes 2 a very ‘odd’ prime, of course. But there are plenty of odd numbers which are not prime; for instance 9,15,81.

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## Copyright information

© Keith Devlin 1984