# Thread: Matrix rings acting on right R-modules - Dauns - Exercies 1-5 no 2

1. ## Matrix rings acting on right R-modules - Dauns - Exercies 1-5 no 2

In Exercise 2 of Exercises 1-5 of John Dauns' book "Modules and Rings" we are given

$R = \begin{pmatrix} \mathbb{Z}_4 & \mathbb{Z}_4 \\ \mathbb{Z}_4 & \mathbb{Z}_4 \end{pmatrix}$ and $M = \overline{2} \mathbb{Z}_4 \times \overline{2} \mathbb{Z}_4$

where the matrix ring R acts on a right R-module whose elements are row vectors.

Find all submodules of M

Helped by Evgeny's post (See Math Help Boards) I commenced this problem as follows:

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Typical elements of R are $R = \begin{pmatrix} r_1 & r_2 \\r_3 & r_4 \end{pmatrix}$

where $r_1, r_2, r_3, r_4 \in \mathbb{Z}_4 = \{ \overline{0}, \overline{1}, \overline{2}, \overline{3} \}$

Typical elements of M are (x,y) where $x, y \in \overline{2} Z_4$

Now the elements of $\overline{2} Z_4 = \overline{2} \times \{ \overline{0}, \overline{1}, \overline{2}, \overline{3} \} = \{ \overline{0}, \overline{2}, \overline{4}, \overline{6} \} = \{ \overline{0}, \overline{2}, \overline{0}, \overline{2} \} = \{ \overline{0}, \overline{2} \}$

Now to find submodules! (Approach is by trial and error - but surely there is a better way!)

Consider a set of the form $N_1 = \{ (x, 0) | \in \overline{2} |mathbb{Z}_4$ - that is $x \in \{ 0, 2 \}$

Let $r \in R$ and then test the action of R on M i.e. $N_1 \times R \rightarrow N_1$ - that is test if $n_1r |in N_1$

Now $(x, 0) \begin{pmatrix} r_1 & r_2 \\r_3 & r_4 \end{pmatrix} = (r_1x, r_2x )$

But now a problem I hope someone can help with!

How do we (rigorously) evaluate $r_1x$ and $r_2x$ and hence check whether $(r_1x, r_2x)$ is of the form (x, 0) [certainly does not look like it but formally and rigorously ...?]

An example of my thinking here

If $r_1 = \overline{3}$ and $x = \overline{2}$ then (roughly speaking!) $r_1 x = \overline{3} \overline{2} = \overline{6} = \overline{2}$

In the above I am assuming that in $\overline{2} \mathbb{Z}_4$ that that $\overline{0}, \overline{4}, \overline{8}, ... = \overline{0}$

and that

that $\overline{2}, \overline{6}, \overline{10}, ... = \overline{2}$

but I am not sure what I am doing here!

Can someone please clarify this situation?

Further, can someone please comment on my overall approach to the Exercise - I am not at all sure regarding how to check for submodules and certainly lack a systematic approach ...

Be grateful for some help ...

Peter

2. ## Re: Matrix rings acting on right R-modules - Dauns - Exercies 1-5 no 2

Deveno,

I accidentally unsubscribed from the thread ... moving too fast ...

Apologies to other members of MHF!

Peter

Deveno,