I've been self-studying some Algebra, and recently did a set of exercises proving both that, for a surjective homomorphism <math xmlns="http://www.w3.org/1998/Math/MathML"> <mi>&#x03D5;<!-- ϕ --></mi> </math> between two groups G and H, that if: (i) G is cyclic, then H is cyclic, and; (ii) G is abelian, then H is abelian.

clasicaacyx

clasicaacyx

Answered question

2022-09-23

Homomorphisms between different types of groups
I've been self-studying some Algebra, and recently did a set of exercises proving both that, for a surjective homomorphism ϕ between two groups G and H, that if:
(i) G is cyclic, then H is cyclic, and;
(ii) G is abelian, then H is abelian.
Proving these two was simple enough, but now I'm thinking about removing the surjectivity constraint on ϕ. It does not seem to me that these statements necessarily hold, in either case, but I am having some difficulty coming up with solid counterexamples to either.
I was considering using the Klein group as my H group for part (i), and the dihedral group of order 3 ( D 3 ) for part (ii), but I was getting stuck here, as I'm not sure precisely to what I should compare it in order to construct . Any hints/suggestions as to where to look would be greatly appreciated.

Answer & Explanation

belidla5a

belidla5a

Beginner2022-09-24Added 8 answers

Step 1
A nice thing to notice is that your choice of group H doesn't really matter - as long as you choose a non-cyclic or non-abelian group respectively, you can form a counterexample.
If we're being really lazy, we can just say "Let G be the trivial group and let f : G H be the map taking the identity to the identity" where the trivial group is the group that only has an identity - and is an example of a cyclic (and therefore abelian) group.
If we're being mildly lazy, we can also let G be whatever group we want and map it into any other group H by sending every element to the identity - this is called the zero map and can send any group to any other one, so there are truly no interesting things one can say simply because two groups have a homomorphism between them, because all pairs of groups do.If we're being a little less lazy and more insightful, we can notice that if H is any group, and x is any element thereof, then the set { , x 2 , x 1 , e , x , x 2 , }, called the subgroup generated by x and noted by ⟨x⟩ is cyclic - and this is basically why cyclic groups are so important. However, ⟨x⟩ has an injective homomorphism into H: just send each element to itself. Thus, as long as you choose H to not have the properties you're asking about, this gives you maps from cyclic groups into it. If you want a more typical cyclic group, like Z/nZ, you can just choose x to have order n and send m∈Z/nZ to x m to create a homomorphism.
Step 2
A more abstract note, which is a theme one definitely learns well in algebra, is that surjective homomorphisms between objects tell you that the range is a quotient of the domain - meaning, basically, that the codomain just the domain with some things identified with each other. This often allows us to determine properties of the codomain from properties of the domain - as the exercise shows.
On the other hand, injective homomorphisms tell you that the domain is a subobject of the range - like how ⟨x⟩ is a subgroup of H. Since we only see a part of the whole picture, usually this means that we can deduce properties of the domain from properties of the codomain - so it works the other way around! (You can check that if f : G H is injective and H is abelian (or cyclic) then G is also abelian (or cyclic))

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