Prime number
A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.
Contents
Description
A natural number greater than 1 that is not a prime number is called a composite number.
For example, 5 is prime because 1 and 5 are its only positive integer factors, whereas 6 is composite because it has the divisors 2 and 3 in addition to 1 and 6.
Central role of primes
The fundamental theorem of arithmetic establishes the central role of primes in number theory: any integer greater than 1 can be expressed as a product of primes that is unique up to ordering.
The uniqueness in this theorem requires excluding 1 as a prime because one can include arbitrarily many instances of 1 in any factorization, e.g., 3, 1 · 3, 1 · 1 · 3, etc. are all valid factorizations of 3.
Primality
The property of being prime (or not) is called primality.
Methods for determined primes
A simple but slow method (see Algorithm) of verifying the primality of a given number n is known as trial division. It consists of testing whether n is a multiple of any integer between 2 and \sqrt{n}.
Algorithms much more efficient than trial division have been devised to test the primality of large numbers. Particularly fast methods are available for numbers of special forms, such as Mersenne numbers. As of April 2014, the largest known prime number has 17,425,170 decimal digits.
History
There are infinitely many primes, as demonstrated by Euclid around 300 BC.
There is no known useful formula that sets apart all of the prime numbers from composites. However, the distribution of primes, that is to say, the statistical behaviour of primes in the large, can be modelled. The first result in that direction is the prime number theorem, proven at the end of the 19th century, which says that the probability that a given, randomly chosen number n is prime is inversely proportional to its number of digits, or to the logarithm of n.
Many questions regarding prime numbers remain open, such as Goldbach's conjecture (that every even integer greater than 2 can be expressed as the sum of two primes), and the twin prime conjecture (that there are infinitely many pairs of primes whose difference is 2). Such questions spurred the development of various branches of number theory, focusing on analytic or algebraic aspects of numbers.
Primes and information technology
Primes are used in several routines in information technology, such as public-key cryptography, which makes use of properties such as the difficulty of factoring large numbers into their prime factors.
Primes and mathematics
Prime numbers give rise to various generalizations in other areas of mathematics, mainly algebra, such as prime elements and prime ideals.
See also
- Algebra
- Algorithm
- Goldbach's conjecture
- Information technology
- Mathematics
- Number
- Number theory
- Prime element
- Prime ideal
- Public-key cryptography
External links
- Prime number @ Wikipedia
