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Homomorphisms!Is this right?

  1. Nov 23, 2008 #1
    1. The problem statement, all variables and given/known data Well, first i appologize for posting problems so often, but i have an exam comming up soon, so i am just working some problems on my own.


    Let G be a cyclic group [a](generated by a). Let b' be any element of a grou p G'.
    (i)Show that ther eis at most one homomorphism from G to G' with [tex] \theta(a)=b'[/tex]
    (ii)Show that there is a homomorphism [tex]\theta[/tex] from G to G' with [tex]\theta(a)=b'[/tex] if and only if the order of b' is an integral divisor of the order of a.
    (iii) state a condition on the orders of a adn b' fro the homomorphism (ii) to be injective.

    2. Relevant equations

    3. The attempt at a solution
    (i) [tex]\theta:G->G'[/tex] [tex] \theta(a)=b'[/tex] I am not sure whether i am getting the queston right. I am assuming that in this case b' would be a fixed element of G' right. ??Because with this in mind, will my reasoning/proof follow below.

    Well, i think that if b'=e' then we will certainly have a homomorphism. SInce

    for any two elements x,y in G, where [tex]x=a^m,y=a^m[/tex] we would have

    [tex]\theta(a^ma^n)=\theta(a^{m+n})=e'=e'e'=\theta(a^m)\theta(a^n)[/tex] so such a mapping would be a group homomorphism between these two groups.
    Now,as in one of my previous questions(which i haven't recieved any answers yet) i am having trouble how to go about proving that we cannot have any other hommomorphism defined by this theta. So how would i prove this?????

    (ii)=> let [tex]\theta:G->G'[/tex] [tex] \theta(a)=b'[/tex] be a homomorphism. Let o(a)=p and o(b')=q.WE want to show that q|p??

    Ok, let [tex]e'=\theta(e)=\theta(a^p)=[\theta(a)]^p=(b')^p=>q|p[/tex]
    <= Let q|p. Now we want to show that [tex]\theta:G->G'[/tex] [tex] \theta(a)=b'[/tex] is a homomorphism.???

    That is we want to show that for any two elements x,y in G, where [tex]x=a^m,y=a^n[/tex] [tex]\theta(xy)=\theta(a^ma^n)=\theta(a^m)\theta(a^n)??[/tex]

    I'm not sure how to go about this one either????
  2. jcsd
  3. Nov 23, 2008 #2


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    Instead of taking b to be identity, just construct the only possible homomorphism. Powers of a must be mapped to powers of b. If you change one of the powers of a to map to a different power of b you no longer have a homomorphism.
  4. Nov 23, 2008 #3
    AHA! So, you are saying to construct my isomorphism for part (i) something like this:

    [tex]\theta(a^i)=b^i[/tex] for i in Z. Well, yeah, i easily showed that this is a homomorphism.

    Now, to prove that this is the oly one in this case, i assumed that the following construction is still a homomorphism

    [tex]\theta(a^i)=b^j[/tex] where i is different from j. Without loss of generality, i supposed that i>j=> there exists an integer k such that i=j+k, so the above mapping would look sth like this:

    [tex]\theta(a^i)=b^{i+k}[/tex] THen i showed that this is not a homomorphism.

    Well thnx for this.
  5. Nov 23, 2008 #4
    Well, for (ii) Here it is again what i think, for <= part.

    SInce q|p=> p=kq for some integer k.

    Now, if we construct a mapping [tex]\theta(a^i)=b^i[/tex] similar to what we did before,( whic i am not sure we an do here too), then we would have:

    [tex]\theta(e)=\theta(a^p)=(b')^p=(b')^kq=(e')^k=e'[/tex] so this means that by this kind of mapping the identity is preserved.

    So, now let x,y be in G. with [tex]x=a^m,y=a^n[/tex] so

    [tex]\theta(xy)=\theta(a^ma^n)=\theta(a^{m+n})=(b')^{m+n}=(b')^m(b')^n=\theta(a^m)\theta(a^n)[/tex] so would this prove it???

    I am not sure this is correct, since i think i didn't use the fact that q|p anywhere in this last part?????? MOreove, i don't even see how to use it...
  6. Nov 23, 2008 #5
    Last edited: Nov 24, 2008
  7. Nov 24, 2008 #6
    Yeah, i think you are right...But, wait untill someone else confirms it..:biggrin:
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