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May 10th, 2017, 10:56 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 <x^3 + 2x + 5 > $ ? 
June 4th, 2017, 08:56 AM  #2 
Member Joined: May 2017 From: Russia Posts: 33 Thanks: 4 
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 nonreal roots. It means field $\displaystyle \mathbb{Q}[x]/<x^3 + 2x + 5>$ is not the splitting field. 

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fields, irreducibility, polynomial, splitting 
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