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: Prove that this is a Group

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    Let A, B be groups and theta: A --> Aut(B) a homomorphism. For a in A denote theta(a)= theta_a in Aut(B). Equip the product set B x A={(b,a): a in A, b in B} with the binary operation (b,a)(b',a')= (b'',a'') where a''=aa' and b''=b(theta_a{b')).

    Show that this binary operation induces a group structure on the set B x A (ie it satisfies the group axioms).

    3. The attempt at a solution

    How do I show that there exists inverses, an identity element and that it is closed? I tried first for identity:

    WTS there exists e such that ae=a=ea. Then I don't know where to go from there. It seems like I am just assuming that there exists e such that aa'=a(a-inverse)=e

    Then from there I can show that there exists an inverse right?
  2. jcsd
  3. Oct 14, 2008 #2
    What is aut(B)?
  4. Oct 14, 2008 #3
    Aut(B) is the set of all automorphisms of G. It is a subgroup of A(G), the set of all 1-1 and onto mappings of G onto itself.
  5. Oct 14, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    With this sort of problem it's usually easiest to simply write down an explicit formula for them. The formula usually isn't too hard to guess, and when it's not obvious, it can usually be determined by solving the appropriate equation.

    Again, direct calculation -- this one is usually trivial.
  6. Oct 14, 2008 #5
    That's where I am stuck--writing the formula. Is the one I wrote with my question completely off?
  7. Oct 14, 2008 #6
    Am I supposed to prove there exists an identity or just that the identity is unique?
  8. Oct 15, 2008 #7
    All you need to prove are the group axioms, which are (besides that the binary operation is closed) associativity and the existence of an identity and inverses. From those you can prove that the identity and inverses are unique.

    Closure is easy: if (b, a) and (b', a') are in B x A, then show that (b, a)(b', a') is also in B x A by direct calculation. Proving associativity is usually tedious but straightforward; remember that θ and θa are homomorphisms.

    For the identity and inverses you can either guess, or write an equation. You could guess (correctly) that the identity is (e, e) (with, of course, the first e being the identity of B and the second from A) and prove that it works, or you could suppose, say, that (be, ae) is the identity, write (b, a)(be, ae) = (b, a), calculate the product, and find ae and be. (Don't forget to prove that the multiplication works the other way, i.e. (be, ae)(b, a) = (b, a).) For inverses, in this case the obvious guess at the inverse of (b, a) doesn't work, but you can still find (b, a)-1 by letting, say, (b, a)-1 = (b*, a*) and writing (b, a)(b*, a*) = (e, e).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook