Prime number
A prime number (or simply prime) is a positive integer 
whose only positive divisors are 1 and itself.
Note that 
is usually defined as being neither prime nor composite because it is its only factor among the natural numbers.
Contents
Identifying primes
The Sieve of Eratosthenes is a relatively quick method for generating a list of the prime numbers.
Importance of Primes
According to the Fundamental Theorem of Arithmetic, there is exactly one unique way to factor a positive integer into a product of primes ( permutations not withstanding). This unique prime factorization plays an important role in solving many kinds of number theory problems.
Famous Primes
Fermat Primes
A Fermat prime is a prime of the form 
. It can easily be shown that for such a number to be prime, n must not have any odd divisor larger than 1 and so must be a power of 2. Therefore all Fermat primes have the form 
. Fermat checked the cases n=0,1,2,3,4 and conjectured that all such numbers were prime. However, 
. In fact, no other Fermat primes have been found.
There is an easy proof of the infinitude of primes based on Fermat numbers (numbers of the form 
). One simply shows that any two Fermat numbers are relatively prime.
Mersenne Primes
A Mersenne prime is a prime of the form 
. For such a number to be prime, n must itself be prime. Compared to other numbers of comparable sizes, Mersenne numbers are easy to check for primality because of the Lucas-Lehmer test.
Twin Primes
Two primes that differ by exactly 2 are known as twin primes. The following are the smallest examples:
3, 5
5, 7
11, 13
17, 19
29, 31
41, 43
It is not known whether or not there are infinitely many pairs of twin primes.
Advanced Definition
When the need arises to include negative divisors, a prime is defined as an integer p whose only divisors are 1, -1, p, and -p. More generally, if R is a unique factorization domain, then an element p of R is a prime if whenever we write 
with 
, then exactly one of a and b is a unit.
