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R is a PID with field of fractions K. $M \subseteq K$ is a fin. generated R-Submodule. I am trying to show M is in fact generated by one element.

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up vote 11 down vote accepted

Consider a finite generating set and find a common denominator for its elements. Now, look at what you've got...

Alternatively, if $M$ is such a submodule, it has no torsion and we know from the structure theorem for f.g. modules over a PID that it must be free. Its rank can be computed by first tensoring with $K$ over $R$ and computing the dimension over $K$ of the resulting $K$-vector space. Because $M$ is contained in $K$, it is very easy to see that that dimension can only be $1$.

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HINT $\: $ Fractions too have $\rm\: gcd\: \bigg(\!\!\dfrac{a}b,\:\dfrac{c}d\!\!\bigg)\ =\ \dfrac{gcd(a,b)}{lcm(b,d)}\ $ $\Rightarrow\:$ Bezoutness $\:\Rightarrow\:$ principality.

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