1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Group Generators

  1. Sep 23, 2007 #1
    1. The problem statement, all variables and given/known data

    Let G be a finite cyclic group of order n. Let a be a generator. Let r be an integer, not zero, and relatively prime to n.

    a)Show that a^r is also a generator of G.

    2. Relevant equations

    3. The attempt at a solution

    For the first one I need to show that for any element b in G there exists some integer m s.t. (a^r)^m = b. I have no idea what to do. The great thing about this is that I will be tested on this tomorrow. Ha, you get it. Pleast try to answer as soon as possible. Thanks for any help.
  2. jcsd
  3. Sep 23, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    b=a^s, since a generates G. Can you solve r*m=s mod n, where r is relatively prime to n. Hint: does r have an inverse in Z(n)?
  4. Sep 23, 2007 #3
    Thanks. But we haven't gotten around Z(n) yet. Don't know what that is. Not sure I'm following you either. Does the solution require knowledge of Z(n) or is there any other way around it? Thanks again.
  5. Sep 24, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    You need to know that if r and n are relatively prime, then there is an integer t such that t*r=1 mod n.
  6. Sep 24, 2007 #5
    oh, so now we multiply both sides of the eqn by s:

    s*t*r = s mod n

    and let m = s*t. Now we get a^(str) = a^(mr) = a^(s mod n) = a^s.

    We have shown for any b in G s.t. b = a^s for some s, b = a^rm for some m.

    Is that good?
  7. Sep 24, 2007 #6
    Wouldn't we have to show also that the period of a^r is n, to make sure that a^r is not generating something with greater size than G?
  8. Sep 24, 2007 #7
    Oh, because a^r is an element of G, it can only generate subgroups which cannot have greater order than n. ok its clear. Thanks a lot.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook