How many isomorphisms are there from $\mathbb{Z}_{18}$ to $\mathbb{Z}_{2} \times \mathbb{Z}_{9}$?
How many isomorphisms are there from $\mathbb{Z}_{18}$ to $\mathbb{Z}_{2} \times \mathbb{Z}_{3} \times \mathbb{Z}_{3}$?
My "solution":
There are $6$ generators of $\mathbb{Z}_{18}$, $6$ generators of $\mathbb{Z}_{9}$, and $1$ generators of $\mathbb{Z}_{2}$. Thus $\mathbb{Z}_{2} \times \mathbb{Z}_{9}$ has $6$ generators. An isomorphism will take generators to generators - there will be $6*6=36$ isomorphism.
Similar strategy.
The solution:
- $6$ isomorphisms
- $\mathbb{Z}_{2} \times \mathbb{Z}_{3} \times \mathbb{Z}_{3}$ is not cyclic thus no isomorphism exists.
I think I see #2 -- $\mathbb{Z}_{2} \times \mathbb{Z}_{3} \times \mathbb{Z}_{3}$, the orders of components are not relatively prime, so they won't combine to a cyclic group. Is that correct?
I however, fail to see #1. Thoughts?