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safe prime numbers

A safe prime is a prime p for which (p - 1)/2 is also prime.

The first few safe prime numbers are: 5, 7, 11, 23, 47, 59, 83, 107, 167, 179, 227, 263, …

4,324 of the first one million positive integers are safe prime.

Learn more about safe prime numbers on wikipedia.

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