Prime Number

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A PrimeNumber is a positive whole number that is divisible only by itself and 1. (and their negatives (of course)).

You have to be a bit careful here. The usual definitions are a bit loose. There are two definitions that are much tighter.

A prime is a positive number p such that if p divides a product ab, then p divides a, or p divides b, or both.

A prime is a positive number p such that it has exactly two positive divisors, itself and 1.

For example 2, 3, 5, 7, 11, ...

Euclid in around 300BC proved that there are infinitely many primes. For one proof that there are infinite primes see Proof By Contradiction.

There are still many things unknown about primes. For example, look at Prime Pairs or Goldbach's Conjecture.

One way of thinking about primes is like this ...

Suppose you have 35 marbles. You can arrange them in a row, but you can also arrange them in a rectangle, 7 by 5.
Suppose you have 28 marbles. You can arrange them in a rectangle, 7 by 4, or 14 by 2.
Suppose now you have 29 marbles. You can't arrange them in a (non-trivial) rectangle, no matter how hard you try. That's because you can't find two positive whole numbers bigger than 1 that multiply to give 29.

One of the uses of prime numbers is in cryptography, especially the RSA Cryptosystem.