# Irreducible but not prime

• Apr 8th 2010, 10:53 PM
hamidr
Irreducible but not prime
I need an example of a ring that is irreducible but not prime.

there is this example of such a ring in the book saying in Z[-(root(3)] where
4= 2*2
but also 4= (1+root(-3))*(1-root(-3))
okay I get it that and I know 1+root(-3) doesn't divide 2 hence is is not prime. but also if it was reducible then we should have units
since the definition of irreducible is that a=b*c then b or c is unit. what is the unit of this ring !!!!?

(ps: if one day I understand everything in abstract algebra, surely I will rewrite the book)
• Apr 8th 2010, 11:44 PM
aliceinwonderland
Quote:

Originally Posted by hamidr
I need an example of a ring that is irreducible but not prime.

there is this example of such a ring in the book saying in Z[-(root(3)] where
4= 2*2
but also 4= (1+root(-3))*(1-root(-3))
okay I get it that and I know 1+root(-3) doesn't divide 2 hence is is not prime. but also if it was reducible then we should have units
since the definition of irreducible is that a=b*c then b or c is unit. what is the unit of this ring !!!!?

You mean an irreducible element but not a prime element in the domain?

You can find such elements in a non-UFD. In UFD, every irreducible element is a prime element though.

Now, take some non-UFD examples.

Consider $D = F[x^2, xy, y^2]$, where F is a field. Then, $(x^2)(y^2) = (xy)(xy)$ (Note: the parenthesis here is not denoted for an ideal ). We see that $(xy) | (x^2)(y^2)$, but it does not imply either $(xy) |(x^2)$ or $(xy) | (y^2)$. Thus xy is not a prime element. However, xy is an irreducible element in D. For example, if $F = \mathbb{Q}, xy = (3xy) \times (1/3)$, then you see that 1/3 is a unit here and xy is not factorized $(x)(y)$ in this domain.

As per your example, $1+\sqrt{-3}$ is not a prime element, but it is an irreducible element. You cannot factorize $1+\sqrt{-3}$ = ab unless a or b is a unit in $\mathbb{Z}[\sqrt{-3}]$.

Quote:

(ps: if one day I understand everything in abstract algebra, surely I will rewrite the book)
(Clapping)
• Apr 9th 2010, 01:21 PM
FancyMouse
Your example (4=2*2=(1+sqrt(-3))(1-sqrt(-3))) works, but the reason is not correct. You should show that neither of them differ by a unit, and everything in the factorization (2,1+sqrt(-3),1-sqrt(-3)) is irreducible.

The unit in Z[sqrt(-3)] is just 1 and -1, since you can define a positive-integer-valued multiplicative "norm" on non-zero elements, and it's easy to show that a unit must have a norm 1, while the only elements in Z[sqrt(-3)]* having norm 1 are 1 and -1.