Theorem: If $G$ is an abelian group such that $H\leq G$, then $H$ is an abelian subgroup.
I've proved this theorem here:
Proof: Suppose $G$ is abelian. Let $a,b\in G$. Then $ab=ba$ as G is abelian and commutative properties are inherited. Since $H\leq G$, then for all $a,b\in H, ab=ba$ also. Thus, $H$ is abelian.
I am new and still learning how to construct proofs -- please be kind!
The question: Is the converse of the theorem true?
I am having a hard time understanding exactly what the text means by just the converse of the theorem. Are they asking "If $G$ is not an abelian group such that $H\leq G$, then $H$ is not an abelian subgroup? Or, are they asking, "If $G$ is an abelian group such that $H\leq G$, then $H$ is not an abelian subgroup?
If $G$ is an abelian group and if $H\leq G$, then can $H$ be an abelian subgroup? Well, isn't this kind of trivial. Suppose $G$ is abelian. Then laws of commutative properties and other group properties follow; that is, there exist an inverse, identity, and $H$ would also be closed under the binary operations *, where the operation * is associative. Then, wouldn't all elements of $H$ also be abelian since they inherit all these properties from $G$?
The converse of a theorem is the statement you obtain when you interchange the hypothesis and the thesis. In this case, the converse should be: "If $G$ is a group such that $H \leq G$ is an abelian subgroup, then $G$ is abelian". You should easily be able to find a counterexample for this claim.