# Prime element

In mathematics, specifically in abstract algebra, a prime element of a commutative ring is an object satisfying certain properties similar to the prime numbers in the integers and to irreducible polynomials. Care should be taken to distinguish prime elements from irreducible elements, a concept which is the same in UFDs but not the same in general.

## Definition

An element $p$ of a commutative ring $R$ is said to be prime if it is not zero or a unit and whenever $p$ divides $ab$ for some $a$ and $b$ in $R$, then $p$ divides $a$ or $p$ divides $b$. Equivalently, an element $p$ is prime if, and only if, the principal ideal $(p)$ generated by $p$ is a nonzero prime ideal.[1]

Interest in prime elements comes from the Fundamental theorem of arithmetic, which asserts that each integer can be written in essentially only one way as 1 or −1 multiplied by a product of positive prime numbers. This led to the study of unique factorization domains, which generalize what was just illustrated in the integers.

Being prime is relative to which ring an element is considered to be in; for example, 2 is a prime element in Z but it is not in Z[$i$], the ring of Gaussian integers, since $2=(1+i)(1-i)$ and 2 does not divide any factor on the right.

## Connection with prime ideals

Main article: Prime ideal

An ideal I in the ring R (with unity) is prime if the factor ring R/I is an integral domain.

A nonzero principal ideal is prime if and only if it is generated by a prime element.

## Irreducible elements

Main article: Irreducible element

Prime elements should not be confused with irreducible elements. In an integral domain, every prime is irreducible[2] but the converse is not true in general. However, in unique factorization domains,[3] or more generally in GCD domains, primes and irreducibles are the same.

## Examples

The following are examples of prime elements in rings:

• The integers ±2, ±3, ±5, ±7, ±11,... in the ring of integers Z
• the complex numbers ($1+i$), 19, and ($2+3i$) in the ring of Gaussian integers Z[$i$]
• the polynomials $x^2 - 2$ and $x^2+1$ in the ring of polynomials over Z.

## References

Notes
1. ^ Hungerford 1980, Theorem III.3.4(i), as indicated in the remark below the theorem and the proof, the result holds in full generality.
2. ^ Hungerford 1980, Theorem III.3.4(iii)
3. ^ Hungerford 1980, Remark after Definition III.3.5
Sources
• Kaplansky, Irving (1970), Commutative rings, Boston, Mass.: Allyn and Bacon Inc., pp. x+180, MR 0254021