[Math] Invariant factors and elementary divisors of an abelian group

abelian-groupsfinite-groups

I have to find the elementary divisors and invariant factors of :

$$ \mathbb Z_6\oplus\mathbb Z_{20}\oplus\mathbb Z_{36}$$

I'm following this.

I think that elementary divisors are $\{2,2^2,2^2,3,3^2,5\}$, just using the prime decomposition of $\{6,20,36\}$.

Using the web I've put above, the invariant factor decomposition is

$$ \mathbb Z_2\oplus\mathbb Z_{12}\oplus\mathbb Z_{180}$$

However, I have written in my notes that the invariant factors are $\{2,2,6,6,30\}$.

I'd like to know which is the right option and where and why I'm wrong.

Thanks in advance.

Best Answer

$\mathbb Z_2\oplus\mathbb Z_{12}\oplus\mathbb Z_{180}$ is right.

Your notes must be wrong because if the invariant factors were $\{2,2,6,6,30\}$ then there wouldn't be an element of order $36$ but $\mathbb Z_6\oplus\mathbb Z_{20}\oplus\mathbb Z_{36}$ has an element of order $36$ coming from $\mathbb Z_{36}$. This also gives elements of order $4$, $9$, $12$, which are not in $\{2,2,6,6,30\}$.