... given instances of unanswered classic questions relate to prime numbers. As an instance of one which does not relate to prime numbers we may refer to the question whether there exists an odd perfect number. A perfect number is a natural number which is equal to the sum of its aliquot parts. Thus 6 is perfect because it is equal to 1 2 3, and 28 is perfect because it is equal to 1 2 4 7 14. Euclid stated a formula which gives all the even perfect numbers, but no one has ever succeeded in proving ... — Popular Science Monthly Volume 86
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