Faster algorithm for primality test

Prime numbers are those positive integers that are divisible only by 1, and itself. For example 2 is a prime number. So are 3, 5, 7, 11, 13, and 17. All of these have exactly two divisors, 1, and the number itself. 1 was considered as a prime for a long time, before it was declared to be not a prime.
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