Irreducible element in the context of "Unique factorization"

In mathematics, an irreducible polynomial is, roughly speaking, a polynomial that cannot be factored into the product of two non-constant polynomials. The property of irreducibility depends on the nature of the coefficients that are accepted for the possible factors, that is, the ring to which the coefficients of the polynomial and its possible factors are supposed to belong. For example, the polynomial x − 2 is a polynomial with integer coefficients, but, as every integer is also a real number, it is also a polynomial with real coefficients. It is irreducible if it is considered as a polynomial with integer coefficients, but it factors as ${\displaystyle \left(x-{\sqrt {2}}\right)\left(x+{\sqrt {2}}\right)}$ if it is considered as a polynomial with real coefficients. One says that the polynomial x − 2 is irreducible over the integers but not over the reals.

Polynomial irreducibility can be considered for polynomials with coefficients in an integral domain, and there are two common definitions. Most often, a polynomial over an integral domain R is said to be irreducible if it is not the product of two polynomials that have their coefficients in R, and that are not unit in R. Equivalently, for this definition, an irreducible polynomial is an irreducible element in a ring of polynomials over R. If R is a field, the two definitions of irreducibility are equivalent. For the second definition, a polynomial is irreducible if it cannot be factored into polynomials with coefficients in the same domain that both have a positive degree. Equivalently, a polynomial is irreducible if it is irreducible over the field of fractions of the integral domain. For example, the polynomial ${\displaystyle 2(x^{2}-2)\in \mathbb {Z} [x]}$ is irreducible for the second definition, and not for the first one. On the other hand, ${\displaystyle x^{2}-2}$ is irreducible in ${\displaystyle \mathbb {Z} [x]}$ for the two definitions, while it is reducible in ${\displaystyle \mathbb {R} [x].}$