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Homework Help: Abstract algebra: irreducible polynomials

  1. May 4, 2008 #1
    1. The problem statement, all variables and given/known data
    Prove that f(x)=x^3-7x+11 is irreducible over Q


    2. Relevant equations



    3. The attempt at a solution
    I've tried using the eisenstein criterion for the polynomial. It doesn't work as it is written so I created a new polynomial g(x)=f(x+1)=(x+1)^3-7(x+1)+11=x^3+3x^2-4x+5. I did this because g(x) and f(x) are similar and if g(x) is irreducible so is f(x), but the new polynomial I constructed doesn't meet the eisenstein criterion either. Any ideas on where I should turn next?
     
  2. jcsd
  3. May 4, 2008 #2
    I've also tried breaking up the polynomial into two smaller ones:
    x^3-7x+11=(x+a)(x^2+bx+c)=x^3+(a+b)x^2+(ab+c)x+ac
    so from this, I get:
    a+b=0; ab+c=-7; ac=11
    by rearranging the equations I get b(-b^2+7)=11, but I don't know where I could go from there to show that no solutions for b exist in Q.
     
  4. May 4, 2008 #3

    Hurkyl

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    Is that a rather severe limitation on a and c?
     
  5. May 4, 2008 #4
    I think I see what you're saying, a and c can't be integers for their product to be 11, but they could both be rationals. I'm trying to prove that the polynomial is irreducible over Q. I do see that when I put b(-b^2+7)=11 into a calculator, my solution is not a rational number. That justifies that it's not reducible to me, but I'm sure there's a more definitive way of showing that without having to say "my calculator says this".
     
  6. May 4, 2008 #5

    Dick

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    If it's reducible over Q then it must have a rational root, since it's a cubic. You might want to use the 'rational root theorem'.
     
  7. May 5, 2008 #6

    Hurkyl

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    Actually, they can. (In exactly 4 different ways)

    But they have to be rational integers! (Because you're factoring an integer polynomial, rather than a more general one with nonintegral rational coefficients)
     
    Last edited: May 5, 2008
  8. May 5, 2008 #7
    hey zero, how do you like abstract algebra so far? i am debating on whether or not to take it this summer. it's a 6-week course and my friend who tutors with me said it's the biggest B you first take :O
     
    Last edited: May 5, 2008
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