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Replaced wrong proof with (I hope!) correct proof.
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Jeremy Rickard
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No.

Let $R$ be the ring of eventually constant sequences of integers, and let $e_i\in R$ be the sequence whose $i^{th}$ term is 1, with all other terms zero.

Then everyif $I$ is the annihilator of an element of a free $R$-module is annihilated by $e_i-e_{i+1}$ for sufficiently large $i$., either

  • there is some $N$ so that $e_i\in I$ for all $i>N$, or
  • there is some $N$ so that $e_i\not\in I$ for all $i>N$.

But the element $x=(e_1,e_2,\dots)$$x=(e_1,e_3, e_5, \dots)$ of the countable product of copies of $R$ is not annihilated by $e_i-e_{i+1}$$e_{2i}$ but not by $e_{2i+1}$ for anyevery $i$, and so the submodule generated by $x$ cannot be a submodule of a free module.

No.

Let $R$ be the ring of eventually constant sequences of integers, and let $e_i\in R$ be the sequence whose $i^{th}$ term is 1, with all other terms zero.

Then every element of a free $R$-module is annihilated by $e_i-e_{i+1}$ for sufficiently large $i$.

But the element $x=(e_1,e_2,\dots)$ of the countable product of copies of $R$ is not annihilated by $e_i-e_{i+1}$ for any $i$, and so the submodule generated by $x$ cannot be a submodule of a free module.

No.

Let $R$ be the ring of eventually constant sequences of integers, and let $e_i\in R$ be the sequence whose $i^{th}$ term is 1, with all other terms zero.

Then if $I$ is the annihilator of an element of a free $R$-module, either

  • there is some $N$ so that $e_i\in I$ for all $i>N$, or
  • there is some $N$ so that $e_i\not\in I$ for all $i>N$.

But the element $x=(e_1,e_3, e_5, \dots)$ of the countable product of copies of $R$ is annihilated by $e_{2i}$ but not by $e_{2i+1}$ for every $i$, and so the submodule generated by $x$ cannot be a submodule of a free module.

Source Link
Jeremy Rickard
  • 35.2k
  • 2
  • 110
  • 151

No.

Let $R$ be the ring of eventually constant sequences of integers, and let $e_i\in R$ be the sequence whose $i^{th}$ term is 1, with all other terms zero.

Then every element of a free $R$-module is annihilated by $e_i-e_{i+1}$ for sufficiently large $i$.

But the element $x=(e_1,e_2,\dots)$ of the countable product of copies of $R$ is not annihilated by $e_i-e_{i+1}$ for any $i$, and so the submodule generated by $x$ cannot be a submodule of a free module.