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Let M be an R-module. We say that M to be Co-generator. If for any family $(M_{i})_{i\in I}$, I is non-empty set, $M_{i}$ is submodule of M such that ${O_{M}}=\cap_{i\in I} M_{i}$ then there is $B\subset I$ is finite that ${O_{M}}=\cap_{i \in B} M_{i}$.

But we have some modules such that any family $(M_{i})_{i\in I}$ if they satisfy ${O_{M}}=\cap_{i \in I} M_{i}$, then there must be $i\in I$ that $M_{i}={0}$:

Example: V is K-vector space with 1-dimensional.

Now, we only consider the modules such that there is a family $(M_{i})_{i\in I}$, I is non-empty set, $M_{i}$ is submodule and $M_{i} \neq {0}$ for all $i\in I$ such that ${O_{M}}=\cap_{i\in I} M_{i}$.

Then my question is :

Whether the following assertion is right?

for any family $(M_{i})_{i\in I}$, I is non-empty set, $M_{i}$ is submodule of M such that ${O_{M}}=\cap_{i\in I} M_{i}$ then there is $B\subset I$ is finite that ${O_{M}}=\cap_{i\in B} M_{i}$ $\Longleftrightarrow$ there is $n\in N$, $x_{1},...,x_{n}\in M$ and $x_{1},...,x_{n} \neq 0$ such that ${O_{M}}=(x_{1}) \cap ... \cap (x_{n})$.

the direction $\Longrightarrow$ is easy by hypothesis:

there is a family $(M_{i})_{i\in I}$, I is non-empty set, $M_{i}$ is submodule and $M_{i}\neq {0}$ for all $i\in I$ such that ${O_{M}}=\cap_{i\in I} M_{i}$

so we choose $x_{i}\in M_{i}$, $x_{i} \neq 0$ then ${O_{M}}=\cap_{i\in I} (x_{i})$, so there is $n\in N$, $x_{1},...,x_{n} \in M$ and $x_{1},...,x_{n} \neq 0$ such that ${O_{M}}=(x_{1})\cap ... \cap (x_{n})$.

Can you help me investigating the inverse direction?.

  • What is $O_M{}$? – Eric Wofsey Oct 24 '15 at 06:08
  • M is albelian, so M has neutral element $0_{M}$. – PHU CUONG LE VAN Oct 24 '15 at 06:10
  • Oh, Now, we consider the module such that there is a family $(M_{i}){i\in I}$, I is non-empty set, $M{i}$ is submodule and $M_{i} \neq {0}$ for all $i\in I$ such that ${O_{M}}=\cap_{i\in I} M_{i}$. And I want to check the above proposotion – PHU CUONG LE VAN Oct 24 '15 at 06:16
  • My mean with the module having a family $(M_{i}){i\in I}$, I is non-empty set, $M{i}$ is submodule and $M_{i} \neq {0}$ for all $i\in I$ such that ${O_{M}}=\cap_{i\in I} M_{i}$. I want to form a equivalent condition for Co-generator. – PHU CUONG LE VAN Oct 24 '15 at 06:19

1 Answers1

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The reverse direction is not true. For instance, let $k$ be a field and consider the ring $R=k[x,y]/(xy)$ as a module over itself. Then $(x)\cap (y)=0$, so the second condition holds. But, for instance, we can define $M_i=(x^i)$, and then $\bigcap M_i=0$ even though no finite intersection of the $M_i$ is $0$.

Eric Wofsey
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