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Math Help - Prove the following statements about the orders of elements...

  1. #1
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    Prove the following statements about the orders of elements...

    L = Z5[x]/(x^2+2x+4) is a field and the multiplicative group L* is generated as a cyclic group by s+3, where s is the congruence class of x modulo x^2+2x+4. Prove the following statements about the orders of elements in L*.

    (a) s has order 12.
    (b) s+1 has order 8.
    (c) s+2 has order 12.
    (d) 2s has order 6.

    I thought this problem wouldn't be so bad but I am having difficulties. Any help would be appreciated!
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  2. #2
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    Re: Prove the following statements about the orders of elements...

    Just use brute force with some shortcuts:
    Prove the following statements about the orders of elements...-orderinfield25.png
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    Re: Prove the following statements about the orders of elements...

    Addendum which somehow got left out.
    For (c), from the list of powers of s, s+2=s11. Since 11 is prime to 12, s+2 has order s, that is 12.
    For (d), 2s=s10. So 2s has order 12/gcd(10,12)=6
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  4. #4
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    Re: Prove the following statements about the orders of elements...

    (a) remember |L*| = 24, so we only need to check orders which are divisors of 24.

    s2 = -2s - 4 = 3s + 1 ≠ 1, so s does not have order 2.

    s3 = s(s2) = s(3s + 1) = 3s2 + s = 3(3s + 1) + s = 4s + 3 + s = 3 (cubes do not always behave as expected in this strange type of field).

    anyway, s3 ≠ 1, so s does not have order 3.

    s4 = (s2)2 = (3s + 1)2 = 4s2 + s + 1 = 4(3s + 1) + s + 1 = 2s + 4 + s + 1 = 3s ≠ 1

    (hmm....it would have easier to calculate s4 = s(s3) = s(3) = 3s. oh well, s doesn't have order 4).

    s6 = (s3)2 = 32 = 4 ≠ 1, s does not have order 6.

    s8 = (s4)2 = (3s)2 = 4s2 = 4(3s + 1) = 2s + 4 ≠ 1, s does not have order 8.

    s12 = (s6)2 = 42 = 1 <---bingo!

    of course you could have calculated ALL the powers of s+3, and made a "discrete log table" (that is: a homomorphism L*-->Z24).

    for example, (s+3)2 = s2 + s + 4 = 3s + 1 + s + 4 = 4s, so:

    log(s+3)(4s) = 2 (our logarithm "base" is s+3, and products in L* get turned into sums in Z24).

    this would allow you to realize any element of L* as (s+3)k (for k = 0,1,2,...,23)

    and then the order of (s+3)k is: 24/gcd(k,24). this puts all the work "up-front" (creating the log table is the most time-consuming), but when finished the order of any element is computed easily:

    |g| = 24/gcd(log(s+3)(g),24)
    Last edited by Deveno; December 29th 2012 at 10:11 PM.
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