# Prime number

In mathematics, a**prime number**, or

**prime**for short, is a natural number greater than 1 whose only positive divisors are 1 and itself. The property of being a prime is called

**primality**. If a number greater than one is not a prime number, it is called a composite number. The sequence of prime numbers begins

- 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113

**P**.

In the context of ring theory, a branch of abstract algebra, the term "prime element" has a specific meaning, and under this meaning, the additive inverse of any prime number is also prime. In other words, when considering the set of integers **Z** as a ring, −7 is a prime element. However, even among mathematicians, the term "prime number" generally means a positive prime integer.

## Representing natural numbers as products of primes

An important fact is the fundamental theorem of arithmetic, which states that every positive integer can be written as a product of primes, and in essentially only one way. Primes are thus the "basic building blocks" of the natural numbers. For example, we can write

## How many prime numbers are there?

There are infinitely many prime numbers. The oldest known proof for this statement is given by the Greek mathematician Euclid in his *Elements* (Book IX, Proposition 20). Euclid states the result as "there are more than any given [finite] number of primes", and his proof can be briefly summarized as follows:

- Take a finite number of primes. Multiply them all together and add one. The resulting number is not divisible by any of the finite set of primes, because dividing by any of these would give a remainder of one. Therefore it must be divisible by some prime that was not included in the finite set.

Even though the total number of primes is infinite, one could still ask "how many primes are there below 100,000" or "How likely is a random 100-digit number to be prime?" Questions like these are answered by the prime number theorem.

## Finding prime numbers

The Sieve of Eratosthenes is a simple way to compute the list of all prime numbers up to a given limit.

In practice though, one usually wants to check if a given number is prime, rather than generate a list of primes. Further, it is often satisfactory to know the answer with a high probability. It is possible to quickly check whether a given large number (say, up to a few thousand digits) is prime using probabilistic primality tests. These typically pick a random number called a "witness" and check some formula involving the witness and the potential prime *N*. After several iterations, they declare *N* to be "definitely composite" or "probably prime". These tests are not perfect. For a given test, there may be some composite numbers that will be declared "probably prime" no matter what witness is chosen. Such numbers are called pseudoprimes for that test. Here's a description of the Fermat primality test.

A new algorithm which determines whether a given number *N* is prime and which uses time polynomial in the number of digits of *N* has recently been described.

## Some properties of primes

- If
*p*is a prime number and*p*divides a product*ab*of integers, then*p*divides*a*or*p*divides*b*. This proposition was proved by Euclid and is known as*Euclid's lemma*. It is used in some proofs of the uniqueness of prime factorizations. - The ring
**Z**/*n***Z**(see modular arithmetic) is a field if and only if*n*is a prime. Put another way:*n*is prime if and only if φ(*n*); =*n*− 1. - If
*p*is prime and*a*is any integer, then*a*^{p}−*a*is divisible by*p*(Fermat's little theorem). - An integer
*p*> 1 is prime if and only if the factorial (*p*− 1)! + 1 is divisible by*p*(Wilson's theorem). Conversely, an integer*n*> 4 is composite if and only if (*n*− 1)! is divisible by*n*. - If
*n*is a positive integer, then there is always a prime number*p*with*n*<*p*≤ 2*n*(Bertrand's postulate). - Adding the reciprocals of all primes together results in a divergent infinite series (proof). More precisely, if
*S*(*x*) denotes the sum of the reciprocals of all prime numbers*p*with*p*≤*x*, then*S*(*x*) = Θ(ln ln*x*) for*x*→ ∞ (see Big O notation). - For each prime number
*p*> 2, there exists a natural number*n*such that*p*= 4*n*± 1. - For each prime number
*p*> 3, there exists a natural number*n*such that*p*= 6*n*± 1. - In every arithmetic progression
*a*,*a*+*q*,*a*+ 2*q*,*a*+ 3*q*,... where the positive integers*a*and*q*≥ 1 are coprime, there are infinitely many primes (Dirichlet's theorem). - The characteristic of every field is either zero or a prime number.
- If
*G*is a finite group and*p*^{n}is the highest power of the prime*p*which divides the order of*G*, then*G*has a subgroup of order*p*^{n}. (Sylow theorems) - If
*p*is prime and*G*is a group with*p*^{n}elements, then*G*contains an element of order*p*. - The prime number theorem says that the number of primes less than
*x*is asymptotic to*x*/(ln*x*).

## Open questions

There are many open questions about prime numbers. For example:

- Goldbach's conjecture: Can every even integer greater than 2 be written as a sum of two primes?
- Twin Prime Conjecture: A
*twin prime*is a pair of primes with difference 2, such as 11 and 13. Are there infinitely many twin primes? - Does the Fibonacci sequence contain an infinite number of primes?
- Are there infinitely many Fermat primes?
- Is there always a prime number between
*n*^{2}and (*n*+ 1)^{2}for every*n*? - Are there infinitely many primes of the form
*n*^{2}+ 1?

## The largest known prime

The largest known prime is 2^{24036583} − 1 (this number is 7,235,733 digits long); it is the 41st known Mersenne prime. M_{24036583} was found on May 15 2004 by a collaborative effort known as GIMPS and it was announced in late May 2004.

The next largest known prime is 2^{20996011} − 1 (this number is 6,320,430 digits long); it is the 40th known Mersenne prime. M_{20996011} was also found on November 17 2003 by a collaborative effort known as GIMPS and announced in early December 2003.

The third largest known prime is 2^{13466917} − 1 (this number is 4,053,946 digits long). It is the 39th known Mersenne prime M_{13466917} also found by GIMPS on November 14 2001 and announced in early December 2001 after double checking. Historically, the largest known prime has almost always been a Mersenne prime since the dawn of electronic computers, because there exists a particularly fast primality test for numbers of this form, the Lucas-Lehmer test.

Some of the largest primes not known to have any particular form (that is, no simple formula such as that of Mersenne primes) have been found by taking a piece of semi-random binary data, converting it to a number `n`, multiplying it by 256^{k} for some positive integer `k`, and searching for possible primes within the interval [256^{k}*n* + 1, 256^{k}(*n* + 1) − 1].
In fact, as a publicity stunt against the Digital Millennium Copyright Act and other WIPO Copyright Treaty implementations, some people have applied this to various forms of DeCSS code, creating the set of illegal prime numbers.
Such numbers, when converted to binary and executed as a computer program, perform acts encumbered by applicable law in one or more jurisdictions.

## Applications

Extremely large prime numbers (that is, greater than 10^{100}) are used in several public key cryptography algorithms. Primes are also used for hash tables and pseudorandom number generators.

## Primality tests

*Main article*

*primality test*

A primality test algorithm is an algorithm which tests a number for primality, i.e. whether the number is a prime number.

## Some special types of primes

A prime *p* is called *primorial* or *prime-factorial* if it has the form *p* = Π(*n*) ± 1 for some number *n*, where Π(*n*); stands for the product 2 · 3 · 5 · 7 · 11 · ... of all the primes ≤ n. A prime is called *factorial* if it is of the form *n* ± 1. The first factorial primes are:

- n! − 1 is prime for n = 3, 4, 6, 7, 12, 14, 30, 32, 33, 38, 94, 166,... (sequence A002982 in OEIS)
- n! + 1 is prime for n = 1, 2, 3, 11, 27, 37, 41, 73, 77, 116, 154... (sequence A002981 in OEIS)

Primes of the form 2^{n} − 1 are known as Mersenne primes, while primes of the form 2^{2n} + 1 are known as Fermat primes. Prime numbers *p* where 2*p* + 1 is also prime are known as Sophie Germain primes. Other special types of prime numbers include Wieferich primes, Wilson primes, Wall-Sun-Sun primes, Wolstenholme primes, unique primes, Newman-Shanks-Williams primes (NSW primes), Smarandache-Wellin primes, Wagstaff primes and supersingular primes.

The base-ten digit sequence of a prime can be a palindrome, as in the prime 10^{31512} + 9700079 · 10^{15753} + 1.

## Prime gaps

For any *N*, the sequence (*N* + 1)! + 2, (*N* + 1)! + 3, ..., (*N* + 1)! + *N* + 1 is a sequence of *N* consecutive composite integers. Therefore, there exist gaps between primes which are arbitrarily large, i.e. for any natural number *N*, there is an integer *n* with *g*_{n} > *N*. (Choose *n* so that *p*_{n} is the greatest prime number less than (*N* + 1)! + 2. On the other hand, the gaps get arbitrarily small in proportion to the primes: the quotient (*g*_{n}/*p*_{n}) approaches zero as *n* approaches infinity.

We say that *g*_{n} is a *maximal gap* if *g*_{m} < *g*_{n} for all *m* < *n*. The largest known maximal gap is 1131, found by T. Nicely and B. Nyman in 1999. It is the 64th smallest maximal gap, and it occurs after the prime 1693182318746371.

Note that the Twin Prime Conjecture simply asserts that *g*_{n} = 1 for infinitely many integers *n*.

## Formulas yielding prime numbers

*Main article*

*formula for primes*

There is no formula for primes which is more efficient at finding primes than the methods mentioned above under "Finding prime numbers". Those which do exist have little practical value.

The curious polynomial *f*(*n*) = *n*^{2} − *n* + 41 yields primes for *n* = 0,..., 40, but *f*(41) is composite. There is no polynomial which only yields prime numbers in this fashion.

There is a set of diophantine equations in 25 variables and one parameter with the following property: the parameter is prime if and only if the resulting system of equations has a solution over the natural numbers. This can be used to obtain a single formula with the property that all its *positive* values are prime.

Another formula is based on Wilson's theorem mentioned above, and generates the number two many times and all other primes exactly once. There are other similar formulae which also produce primes.

## Generalizations

In number theory itself, one talks of "probable primes", integers which, by virtue of having passed a certain test, are considered to be probably prime. Probable primes which are in fact composite (such as Carmichael numbers) are called pseudoprimes.

One can define prime elements and irreducible elements in any integral domain. For the ring of integers, the set of prime elements equals the set of irreducible elements; it's {...−11, −7, −5, −3, −2, 2, 3, 5, 7, 11, ...}.

As another example, we can extend the integers to the Gaussian integers **Z**[*i*], that is, complex numbers of the form *a* + *bi* with *a* and *b* in **Z**. This is an integral domain, and its prime elements are the Gaussian primes. Note that 2 is not a Gaussian prime, because it factors into the product of the two Gaussian primes (1 + *i*) and (1 − *i*). The element 3, however, remains prime even in the Gaussian integers. In general, rational primes (i.e. prime elements in the ring of integers) of the form 4*k* + 3 are Gaussian primes, whereas rational primes of the form 4*k* + 1 are not.

In ring theory, one generally replaces the notion of *number* with that of ideal.
*Prime ideals* are an important tool and object of study in commutative algebra, algebraic number theory and algebraic geometry.
The prime ideals of the ring of integers are the ideals (0), (2), (3), (5), (7), (11), ...

In class field theory yet another generalization is used. Given an arbitrary field *K*, one considers valuations on *K*, certain functions from *K* to the real numbers **R**. Every such valuation yields a topology on *K*, and two valuations are called *equivalent* if they yield the same topology. A *prime of K* is an equivalence class of valuations. With this definition, the primes of the field **Q** of rational numbers are represented by the standard absolute value function (known as the "infinite prime") as well as by the *p*-adic valuations on **Q**, for every prime number *p*.

## Quotes

- "Mathematicians have tried in vain to this day to discover some order in the sequence of prime numbers, and we have reason to believe that it is a mystery into which the human mind will never penetrate." —
*Leonhard Euler* - "God may not play dice with the universe, but something strange is going on with the prime numbers." —
*Paul Erdos*

## Prime number fun

- Prime curios at the prime pages
- The Prime Number Shitting Bear is a web cartoon

## Books

- Karl Sabbagh,
*The Riemann Hypothesis: The Greatest Unsolved Problem in Mathematics*. Farrar, Straus and Giroux; 340 pages - John Derbyshire,
*Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics*. Joseph Henry Press; 448 pages - Marcus du Sautoy,
*The Music of the Primes: Searching to Solve the Greatest Mystery in Mathematics*. HarperCollins; 352 pages - H. Riesel,
*Prime Numbers and Computer Methods for Factorization*, 2nd ed., Birkhäuser 1994.

## External links

- The prime pages -- http://www.utm.edu/research/primes/
- MacTutor history of prime numbers
- The "PRIMES is in P" FAQ
- Prime Numbers the first 20,000 primes at Wikisource.
- The prime puzzles
- The Prime Project generates a new prime number every time the page is loaded
- An English translation of Euclid's proof that there are infinitely many primes
- Primes from WIMS is an online prime generator.