# Irreducible element

In abstract algebra, a non-zero non-unit element in an integral domain is said to be **irreducible** if it is not a product of two non-units.

## Relationship with prime elements

Irreducible elements should not be confused with prime elements. (A non-zero non-unit element in a commutative ring is called prime if, whenever for some and in , then or .) In an integral domain, every prime element is irreducible,^{[1]}^{[2]} but the converse is not true in general. The converse *is* true for UFDs^{[2]} (or, more generally, GCD domains.)

Moreover, while an ideal generated by a prime element is a prime ideal, it is not true in general that an ideal generated by an irreducible element is an irreducible ideal. However, if is a GCD domain, and is an irreducible element of , then the ideal generated by *is* a prime ideal of .^{[3]}

## Example

In the quadratic integer ring , it can be shown using norm arguments that the number 3 is irreducible. However, it is not a prime element in this ring since, for example,

but does not divide either of the two factors.^{[4]}

## See also

## References

- ↑ Consider p a prime that is reducible: p=ab. Then p | ab => p | a or p | b. Say p | a => a = pc, then we have: p=ab=pcb => p(1-cb)=0. Because R is an integral domain we have: cb=1. So b is a unit and p is irreducible.
- ↑
^{2.0}^{2.1}Sharpe (1987) p.54 - ↑ http://planetmath.org/encyclopedia/IrreducibleIdeal.html
- ↑ William W. Adams and Larry Joel Goldstein (1976),
*Introduction to Number Theory*, p. 250, Prentice-Hall, Inc., ISBN 0-13-491282-9

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