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If $I$ is an ideal in a ring $R$ let $[R:I] =\{r \in R\mid xr \in I\text{ for every }x \in R\}$. How can I show that $[R:I]$ is an ideal of $R$ which contains $I$.

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    Welcome to math.SE: since you are a new user, I wanted to let you know a few things about the site. In order to get the best possible answers, it is helpful if you say what your thoughts on it are so far; this will prevent people from telling you things you already know, and help them write their answers at an appropriate level.2012-04-27
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    Hint: First show that I is contained in here (use the fact that I is an ideal). Then the ideal part should follow immediately, since for $r\in [R : I]$ and any $x\in R$ we have $xr \in I \subset [R:I]$2012-04-27
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    You have swapped the "numerator" and the "denominator" in your definition of what is sometimes called a colon ideal. The correct notation is $$ (I:R) =\{r \in R\mid xr \in I\text{ for every }x \in R\} $$2012-04-27

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