# Wrong(?) Question about Irreducibiliity of Polynomials

• Apr 26th 2011, 05:15 AM
slevvio
Wrong(?) Question about Irreducibiliity of Polynomials
Here is a question from my Galois Theory course:

Let f be a polynomial over \$\displaystyle \mathbb{Z}\$ with degree 2 or 3, and suppose it is reducible over \$\displaystyle \mathbb{Q}\$. What is wrong with the following argument: f is reducible over \$\displaystyle \mathbb{Z}\$ by Gauss's Lemma and so must have a zero over \$\displaystyle \mathbb{Z}\$ [Give both a counterexample and show where the error in the proof is].

But wait a minute! Isn't the statement above true by the contrapositive of Gauss's Lemma (i.e. f irreducible over Z => f irreducible over Q)? And isn't it true anyway since the ring of integers of Q is Z, and hence by definition since f has coefficients in Z and is factorisable, its roots will lie in the algebraic integers, which are the integers ? Thanks if anyone could help me with this! :)
• Apr 26th 2011, 06:27 AM
Deveno
what if f(x) = 4x^2 + 12x + 9?

clearly f(x) factors over Z as (2x + 3)^2

are the roots to this equation in Z?
• Apr 29th 2011, 02:53 PM
slevvio
Sorry - Gauss' Lemma doesn't tell you about the roots! oops