Prime number

A prime number (or simply prime) is a positive integer $p>1$ whose only positive divisors are 1 and itself. Note that $1$ is usually defined as being neither prime nor composite because it is its only factor among the natural numbers.

Famous Primes

Fermat Primes

A Fermat prime is a prime of the form $2^n+1$ . It can easily be shown that for such a number to be prime, n must be a power of 2. Therefore Fermat primes can also be written as $2^{2^n}+1$ . Fermat checked the cases n=0,1,2,3,4 and conjectured that all such numbers were prime. However, $2^{2^5}+1=641\cdot 6700417$ . In fact, no other Fermat primes have been found.

There is an easy proof of the infinitude of primes based on Fermat numbers. One simply shows that any two Fermat numbers are relatively prime.

Mersenne Primes

A Mersenne prime is a prime of the form $2^n-1$ . 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. A natural attempt to prove that there are infinitely many twin primes is to consider the sum of reciprocals of all the twin primes $B=\frac{1}{3}+\frac{1}{5}+\frac{1}{5}+\frac{1}{7}+\frac{1}{11}+\frac{1}{13}+\frac{1}{17}+\frac{1}{19}+\cdots$ . If $B=\infty$ , then there would be infinitely many twin primes. However, it turns out that $B<\infty$ , which proves nothing. The number B is called Brun's constant.

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 $p=ab$ with $a,b\in R$ , then exactly one of a and b is a unit.

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