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 May 10th, 2017, 09:56 PM #1 Member     Joined: Oct 2014 From: Colorado Posts: 40 Thanks: 21 Irreducibility and splitting fields of a polynomial If i'm given a polynomial $x^3 + 2x + 5$ how can I tell if it's irreducible over $\mathbb{Q}[x]$? I've tried Eisenstiens irreducibility criterion with no luck. Also if we can't factor a polynomial, is its splitting field just $\mathbb{F} \backslash$ ?
 June 4th, 2017, 07:56 AM #2 Member   Joined: May 2017 From: Russia Posts: 34 Thanks: 5 Hello! If the polynomial were reducible over $\displaystyle \mathbb{Q},$ it would be reducible over $\displaystyle \mathbb{Z}.$ And it would be reducible modulo $\displaystyle 3$. But it doesn't have a root in $\displaystyle \mathbb{F}_3.$ That means it is irreducible over $\displaystyle \mathbb{Q}.$ Using a computer one can see that it has one real root and two complex non-real roots. It means field $\displaystyle \mathbb{Q}[x]/$ is not the splitting field.

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