transfinite arithmetic, cardinal arithmetic, ordinal arithmetic
prime field, p-adic integer, p-adic rational number, p-adic complex number
arithmetic geometry, function field analogy
A prime number is that which is measured [=divided by smaller number] by a monad [=unit] alone.
[Euclid, Def. 11 of Elements Book VII (~ 400-300 BC), see here]
A prime number is a natural number greater than 1 that cannot be written as a product of finitely many natural numbers (all) other than itself, hence a natural number greater than 1 that is divisible only by 1 and itself.
Equivalently, a prime number is a natural number such that is not equal to 1, and for all natural numbers and , implies that either is equal to 1 or is equal to 1.
This means that every natural number is, up to re-ordering of factors, uniquely expressed as a product of a tuple of prime numbers:
This is called the prime factorization of , proved in the fundamental theorem of arithmetic.
Notice that while the number is, clearly, only divisible by one and by itself, hence might look like it deserves to be counted as a prime number, too, this would break the uniqueness of this prime factorization. In view of the general phenomenon in classifications in mathematics of some objects being too simple to be simple one could say that 1 is “too prime to be prime”.
However, historically, some authors did count 1 as a prime number, see e.g. Roegel 11.
A number is prime if and only if it generates a maximal ideal in the rig of natural numbers.
Prime numbers do not quite match the prime elements of , since generates a prime ideal but not a maximal ideal; instead they match the irreducible elements (Wikipedia).
From the Isbell-dual point of view, where a commutative ring such as the integers is regarded as the ring of functions on some variety, namely on Spec(Z), the fact that prime numbers correspond to maximal ideals means that they correspond to the closed points in this variety (see this Example), one also writes
This dual perspective on number theory as being the geometry (algebraic geometry) over Spec(Z) is called arithmetic geometry.
see at Riemann hypothesis
see at Goldbach conjecture
see at prime number theorem
The following statements are equivalent for a finite set :
has a prime number cardinality
is not a singleton, and for all finite sets and for which there exists a bijection , either is a singleton or is a singleton.
In dependent type theory, this is expressed for as
For historical discussion see
See also
Wikipedia, Prime number
Neil Sloane, Sequence A000040 (The prime numbers.), The On-Line Encyclopedia of Integer Sequences, OEIS Foundation [web]
Last revised on May 18, 2025 at 15:54:57. See the history of this page for a list of all contributions to it.