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This is a problem from Aluffi's book, chapter V 2.17.

"Let $R$ be a Euclidean Domain that is not a field. Prove that there exists a nonzero, nonunit element $c$ in $R$ such that $\forall a \in R$, $\exists q$, $r \in R$ with $a = qc + r$, and either $r = 0$ or $r$ a unit."

Ok, I know that if $c\mid a$ then $r=0$, but if $c\nmid a$, not sure about what to do. I took the classic Euclidean Domain $\mathbb{Z}$ as example, and in $\mathbb{Z}$ I know that $c = 2$ ( also $-2$). Then I tried to generalize this.

I did $c = unit + unit$, but this didn't help and exercise 2.18 showed me that $c$ is not always $unit+unit$. I'm out of ideas, need some help.

Thanks.

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    So Aluffi doesn't use the terminology of *universal side divisor*. Interesting.2017-02-11

2 Answers 2

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If $R$ is not a field, then it has nonunits. Consider the set $S=\{\varphi(a)\mid a\text{ is not a unit, and }a\neq 0\}$, where $\varphi$ is the Euclidean function.

It is a nonempty set of positive integers. By the Least Element Principle, it has a smallest element. Let $c\in R$ be a nonunit, nonzero, such that $\varphi(c)$ is the smallest element of $S$.

Edited. I claim that $c$ satisfies the conditions of the problem. Let $a\in R$. Then we can write $a = qc + r$, with either $r=0$ or $\varphi(r)\lt \varphi(c)$. If $r=0$, we are done. If $r\neq 0$, then $\varphi(r)\lt \varphi(c)$, then $\varphi(r)\notin S$, hence $r$ does not satisfy the condition

$r$ is not a unit, and $r\neq 0$.

Since $r\neq 0$, it follows that $r$ must be a unit.

Thus, for every $a\in R$, there exist $q,r\in R$ such that $a=qc+r$, and either $r=0$ or $r$ is a unit, as desired.

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    In that case we have $a = qc + r$, with $r = 0$ or $\varphi(r)\leq\varphi(c)$ for all $a \in R$. Using the Least Element Principle i know that $r = 0$ or $\varphi(r) = \varphi(c)$, cant be smaller.2012-06-15
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    @Integral: The definition of "Euclidean domain" *requires* that the $q$ and $r$ satisfy $r=0$ or $\varphi(r)\lt\varphi(c)$; must be *strictly* smaller, cannot be equal. So either $r=0$ or $\varphi(r)\lt \varphi(c)$. Also, notice that $\varphi(c)$ is not the smallest value that $\varphi$ can take **overall**, it's just the smallest value that it can take *when the input is not a unit*.2012-06-15
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    ops... In that case we have a=qc+r, with r=0 or φ(r)<φ(c) for all a∈R. Using the Least Element Principle i know that r=0 , cant be smaller. But how this let me know that r could be 1 ?2012-06-15
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    @Integral: **no.** You have the definition of Euclidean domain wrong. And you are not looking carefully at the definition of $S$. There is no *a priori* reason why something cannot have $\varphi$ smaller than $\varphi(c)$. Look **carefully** at the definition of $S$, don't just repeat the same false statement you did three minutes ago.2012-06-15
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    Yes, my fault. I need some time to process.2012-06-15
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    Let me try again. Assumig $a$ is not a unit and $\varphi(c)$ is the smaller element of $S$, we have that $a=qc+r$ implies $r = 0$, because can't be $\varphi(r) < \varphi(c)$. If $a$ is a unit, we have $a=1=qc+r$, but $c$ is not a unit, so can't be $1=qc+0$, in that case $1=0\cdot c+1$ and $r=1$. **im still not sure about "$c$ is not a unit, so can't be $1=qc+0$", i'll check this. But is this the right way? Am i doing something wrong again? Sorry, i started to see this yesterday, still not used to this definitions.2012-06-15
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    If $a$ is a unit and $a=qc+0$ then $1=qc$, so $c$ is a unit. Im confusing 1 and unit, is not the same. Need more carefull reading. Thanks for the help!2012-06-15
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    @Integral: No, no, **no**. Again: $S$ is only the values of $\varphi$ at nonzero **nonunits**. So $a=qc+r$ with $r=0$ or $\varphi(r)\lt \varphi(c)$. If $r\neq0$, then $\varphi(r)\lt \varphi(c)$, so $\varphi(r)$ is not in $S$, so $r$ does not satisfy "$r$ is not a unit and $r\neq 0$", so either $r$ is a unit or $r=0$, so $r$ is a unit.2012-06-15
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    @Integral: Also, it is false that if $a$ is a unit then it is equal to $1$.2012-06-15
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    I understand, you are right. thank you again, more than just answering my question, you showed me that i need to be more carefull about definitions. I really need to read all again.2012-06-15
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Try an element $c$ of smallest value greater than $1$. Then division by $c$ has a remainder of zero or value $1$, which is therefore an unit.