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I've just started studying this topic and I've stopped at this exercise: "Let $M = \mathbb{Z}^3$ and $N$ the submodule generated by $\{(1,1,6),(1,-1,6)\}$. Determine a basis of $\{v_1,v_2,v_3\}$ of $M$ an integer $s \geq 0$ and $d_1, ..., d_s \in \mathbb{Z}$ with $\langle d_1\rangle \subset ... \subset \langle d_s\rangle$ such that $\{d_1v_1, ... , d_sv_s\}$ is a basis of $N$. Determine the invariants and elementary divisors of $M/N$ and the rank of $(M/N)/T(M/N)$.

The first part (I think) it is clear. I realized that $\langle\{(1,1,0),(1,-1,0),(0,0,1)\}\rangle = \mathbb{Z}^3$ and, also, that if I define $v_1=(1,-1,0)$ and $v_2 = (0,0,1)$, we have that $\langle\{1.v_1,6.v_2\}\rangle = N$. But, I'm stuck in the rest. Could anyone help?

Anyway142
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1 Answers1

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We have that $\langle\{(1,1,0),(1,-1,0),(0,0,1)\}\rangle = \mathbb{Z}^3$ and, thus, for $s=2$ and $v_2= (1,-1,0), v_1 = (0,0,1)$ and $d_1=6, d_1=1$, we have that $\langle\{d_1v_1,d_2v_2\}\rangle = N$, with $\langle d_1\rangle=6\mathbb{Z} \subset \langle d_2\rangle = 1\mathbb{Z}$.

Inr order to find $M/N$, first of all define $N:A\mathbb{Z}^3$, where $A$ is the matrix \begin{equation} A = \left( \begin{array} {cc} 1& 1 \\ 1 &-1 \\ 6& 6\nonumber \end{array} \right) \end{equation}

We can use rows and colums operations to find a similar matrix $A'$ \begin{equation} A'= \left( \begin{array} {cc} 0& 1 \\ 0 &-6 \\ 2& 0\nonumber \end{array} \right) \end{equation}

And, clearly, we have that $A = \mathbb{Z} \times 6\mathbb{Z} \times 2\mathbb{Z}$. This way, \begin{equation} M/N = \frac{\mathbb{Z}^3}{\mathbb{Z} \times 6\mathbb{Z} \times 2\mathbb{Z}} \simeq \mathbb{Z}_6 \times \mathbb{Z}_2 \simeq \mathbb{Z}/6\mathbb{Z} \times \mathbb{Z}/2\mathbb{Z} \nonumber \end{equation}

and, thus, the invariants are $2\mathbb{Z} = \langle 2 \rangle$ and $6\mathbb{Z} = \langle 6 \rangle$ and the elementary divisors are $2$ and $6$.

In order to find the rank of $(M/N)/T(M/N)$, we note that $T(M/N) = \{(x,y) \in M/N | (x,y).z = 0 \textrm{ for a regular } z \in \mathbb{Z}\}$ which is, in other words, an element $z$ such that $xz \in 6\mathbb{Z}$ and $yz \in 2\mathbb{Z}$, and since for $z \in 6\mathbb{Z}$, every element of $M/N$ would be in $T(M/N)$ and, so, $T(M/N) = M/N$, which implies that $(M/N)/T(M/N) = \{0\}$, whose rank is $1$.

Anyway142
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