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Homework Help: Finding inverse in polynomial factor ring

  1. Feb 2, 2015 #1
    1. The problem statement, all variables and given/known data
    find the inverse of r in R = F[x]/<h>.
    r = 1 + t - t^2
    F = Z_7 (integers modulo 7), h = x^3 + x^2 -1

    2. Relevant equations

    3. The attempt at a solution
    The polynomial on bottom is of degree 3, so R will look like:
    R = {a + bt + ct^2 | a,b,c are elements of z_7 and x^3 = 1 - ^2}

    To solve this problem I realized that the inverse must obviously have the form of some element in R, so I set up:
    (a + bt + ct^2)(1 + t - t^2) = 1

    then I multiplied it all out whilst continuously substituting for t^3 and then solving for coefficients where the constant coefficient should equal 1 and the other two should equal 0.

    I did all of this and got the constant coefficient to be zero and nonzero answers for the other two >.<. I checked my calculations and can't find an error (doesn't necessarily mean there isn't one...), is something wrong with the way I set up the problem? is my substitution for x^3 correct?
  2. jcsd
  3. Feb 2, 2015 #2


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    Homework Helper

    The usual way to do this is to use the Extended Euclidean Algorithm to explicitly write Bezout's identity. gcd(h,r)=a*h+b*r. Then mod both sides by h. Does that sound familiar? It is kind of a tedious calculation and it's easy to make a mistake. What did you do with t^4?
    Last edited: Feb 2, 2015
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