# Prime Number

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A prime number is a positive whole number that is divisible only by itself and 1.

Well, almost. A prime is also divisible by the negative of itself, and -1, but we usually don't mention that. So 29 is a prime because the only numbers that divide evenly into 29 are 1, 29, -1 and -29. On the other hand, 28 is not prime because it's divisible by lots of other numbers, including 2, 4 and 7.

Examples of primes are 2, 3, 5, 7, 11, 13, 17, ...

In around 300BC a guy called Euclid proved that there are infinitely many primes, but there are still many things unknown about them. For example, here are some primes that differ by exactly two:

• 11 and 13
• 29 and 31
• 101 and 103

Are there infinitely many such pairs? No one knows ...

One of the Millennium Problems in mathematics, namely the Riemann Hypothesis, relates to prime numbers and their distribution.

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