Homomorph-containing not implies no nontrivial homomorphism to quotient group
This article gives the statement and possibly, proof, of a non-implication relation between two subgroup properties. That is, it states that every subgroup satisfying the first subgroup property (i.e., homomorph-containing subgroup) need not satisfy the second subgroup property (i.e., normal subgroup having no nontrivial homomorphism to its quotient group)
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Statement
It is possible to have a group and a subgroup such that:
- is a homomorph-containing subgroup in , i.e., for any homomorphism in the image of is contained in .
- is not a normal subgroup having no nontrivial homomorphism to its quotient group. In other words, there exists a nontrivial homomorphism from to .
Proof
Take the following:
- is cyclic group:Z4.
- is the subgroup Z2 in Z4 (hence isomorphic to cyclic group:Z2), which is the unique subgroup generated by elements of order two.
Then:
- is homomorph-containing in : This is because the image of under any homomorphism must also have exponent at most two, hence must be inside .
- There is a nontrivial homomorphism from to : In fact, both are isomorphic to cyclic group:Z2.