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Thread: Determine All Ring Homomorphisms: How to tackle such problems?

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    Junior Member rubix's Avatar
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    Determine All Ring Homomorphisms: How to tackle such problems?

    Let me start by saying i have missed pretty much the entire lecture on Group Homomorphism and probably more than a half of Ring Homomorphism so go easy on me.

    I'm trying to catch up from the book...trying to do as much exercise as i can but i'm stuck on the very basic idea. How to determine all Group and Ring Homomorphism for a given mapping.

    I know the properties of Homomorphism, theorems/lemmas related to it...and atm i can do problems like "given mapping, prove or disprove it is ring homomorphism".

    Anyway, how does one go aboot approaching such problems? Am i supposed to use 1-> 1 property, if so how?

    Here are some examples:

    { i would really appreciate if you gave a through explanation (along with your approach if possible) rather than just the solution }

    1a) Determine all homomorphisms from Z_12 to Z_30

    1b) Determine all ring homomorphisms from Z_12 to Z_30

    2) Determine all ring homomorphisms from Z/6 to Z/2

    3) Determine all ring homomorphisms from Q -> Q

    4) Determine all ring homomorphism from R to R

    Much appreciated!
    Last edited by rubix; Nov 17th 2009 at 06:44 AM.
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    Quote Originally Posted by rubix View Post
    Let me start by saying i have missed pretty much the entire lecture on Group Homomorphism and probably more than a half of Ring Homomorphism so go easy on me.

    I'm trying to catch up from the book...trying to do as much exercise as i can but i'm stuck on the very basic idea. How to determine all Group and Ring Homomorphism for a given mapping.

    I know the properties of Homomorphism, theorems/lemmas related to it...and atm i can do problems like "given mapping, prove or disprove it is ring homomorphism".

    Anyway, how does one go aboot approaching such problems? Am i supposed to use 1-> 1 property, if so how?

    Here are some examples:

    { i would really appreciate if you gave a through explanation (along with your approach if possible) rather than just the solution }

    1a) Determine all homomorphisms from Z_12 to Z_30

    1b) Determine all ring homomorphisms from Z_12 to Z_30

    2) Determine all ring homomorphisms from Z/6 to Z/2

    3) Determine all ring homomorphisms from Q -> Q

    Much appreciated!

    I won't do these problems "thoroughly" but only remind you some very basic facts since, as you say, you already know the elementary stuff around this

    subject: take problem (1) for instance, and let $\displaystyle f:\mathbb{Z}_{12}\rightarrow\mathbb{Z}_{30}$ be a ring homom., then $\displaystyle f\left(\mathbb{Z}_{12}\right)<\mathbb{Z}_{30}\Long rightarrow f\left(\mathbb{Z}_{12}\right)$ must divide 30 AND ALSO 12(for example, because

    any such ring homom. is also a group homom. and thus by Lagrange's theorem).

    Now, since $\displaystyle \mathbb{Z}_{12}=\,<a>$ as an additive group (meaning: every element of the ring is of the form $\displaystyle na$ for some $\displaystyle n\in\mathbb{Z}$ and, in fact, we can choose $\displaystyle 0\leq n\leq 11$ since this

    is just the ring of residues modulo 12), then $\displaystyle f$ is uniquely and completely determined by its value on $\displaystyle a$, so you've to count all the possible choices for the

    image $\displaystyle f(a)\,\,o\!f\,\,a\,\,in\,\,\mathbb{Z}_{30}$ and s.t. the subgroup $\displaystyle <f(a)>\,\in\,\mathbb{Z}_{30}$ has order dividing 30 and 12...

    For example, if $\displaystyle \mathbb{Z}_{30}=\,<b>$ as an additive group, then it CAN NOT BE that $\displaystyle f(a)=b\,\,\,or\,\,\,f(a)=7b$, since both $\displaystyle b\,,\,7b$ generate the whole $\displaystyle \mathbb{Z}_{30}$, but $\displaystyle f(a)=5b$ is fine since $\displaystyle \left|<5b>\right|=6$....etc.

    Tonio
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