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Homework Help: Why is this extension normal?

  1. Dec 6, 2016 #1
    1. The problem statement, all variables and given/known data
    I'm following the solutions to a homework tutorial and I'm having trouble understanding why what they're saying is true.

    Question: Let f be a polynomial in K[x] and let S be the splitting field of f over K. decide whether the extension S:K is galois and describe the structure of the Galois group Gal(S:K) for the polynomial x^3+2x+1 over ##F_3##.

    2. Relevant equations

    3. The attempt at a solution
    S:F_3 will be a galois extension because the polynomial is separable and S is a finite splitting field for f over F_3.

    Now I tried to construct the splitting field of F_3 to figure out what S looks like by first noticing that f has no roots in F_3, I then adjoined a root r to F_3. My notes say that "F_3(r):F_3 is normal and so all the roots of the minimal polynomial are in F_7(r)." I'm quite confused on how they know this extension is normal. I get that S:F is Galois, but for any intermediate field K such that S>K>F the extension K>F is not gaurentee'd to be normal. How do they know F_3(r):F_3 is normal?
  2. jcsd
  3. Dec 7, 2016 #2


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    Staff: Mentor

    Why ##\mathbb{F}_7##? You change the characteristic this way. If you have a root ##r##, why don't you calculate the other two roots of ##f(x)=x^3+2x+1##?
  4. Dec 8, 2016 #3
    I'm sorry, I don't know why I keep typing ##F_7## >.<
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