Prime Number
Checks whether a number is prime and finds nearby primes.
Prime?
Yes
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Methodology
A number is prime if it has exactly two divisors: 1 and itself.
Example: 17 is prime, 18 is not (18 = 2 × 9)
Methodology
A prime divides only 1 and itself; test divisibility up to √n.
Example
17 is prime, 15 is not (3×5).