# any divisor is gcd of two principal divisors

Using the exponent valuations, one can easily prove the

In any divisor theory, each divisor is the greatest common divisor of two principal divisors.

Proof.  Let  $\mathcal{O}^{*}\to\mathfrak{D}$  be a divisor theory and $\mathfrak{d}$ an arbitrary divisor in $\mathfrak{D}$.  We may suppose that $\mathfrak{d}$ is not a principal divisor (if $\mathfrak{D}$ contains exclusively principal divisors, then  $\mathfrak{d}=\gcd(\mathfrak{d},\,\mathfrak{d})$  and the proof is ready).  Let

 $\mathfrak{d}=\prod_{i=1}^{r}\mathfrak{p}_{i}^{k_{i}}$

where the $\mathfrak{p}_{i}$’s are pairwise distinct prime divisors and every $k_{i}>0$.  Then third condition in the theorem concerning divisors and exponents allows to choose an element $\alpha$ of the ring $\mathcal{O}$ such that

 $\nu_{\mathfrak{p}_{1}}(\alpha)=k_{1},\;\;\ldots,\;\;\nu_{\mathfrak{p}_{r}}(% \alpha)=k_{r}.$

Let the principal divisor corresponding to $\alpha$ be

 $(\alpha)=\prod_{i=1}^{r}\mathfrak{p}_{i}^{k_{i}}\prod_{j=1}^{s}\mathfrak{q}_{j% }^{l_{j}}=\mathfrak{dd}^{\prime},$

where the prime divisors $\mathfrak{q}_{j}$ are pairwise different among themselves and with the divisors $\mathfrak{p}_{i}$.  We can then choose another element $\beta$ of $\mathcal{O}$ such that

 $\nu_{\mathfrak{p}_{1}}(\beta)=k_{1},\;\;\ldots,\;\;\nu_{\mathfrak{p}_{r}}(% \beta)=k_{r},\;\;\nu_{\mathfrak{q}_{1}}(\beta)=\ldots=\nu_{\mathfrak{q}_{s}}(% \beta)=0.$

Then we have  $(\beta)=\mathfrak{dd}^{\prime\prime}$,  where  $\mathfrak{d}^{\prime\prime}\in\mathfrak{D}$  and

 $\gcd(\mathfrak{d}^{\prime},\,\mathfrak{d}^{\prime\prime})=\mathfrak{q}^{0}% \cdots\mathfrak{q}^{0}=\mathfrak{e}=(1).$

The gcd of the principal divisors $(\alpha)$ and $(\beta)$ is apparently $\mathfrak{d}$, whence the proof is settled.

Title any divisor is gcd of two principal divisors AnyDivisorIsGcdOfTwoPrincipalDivisors 2013-03-22 17:59:37 2013-03-22 17:59:37 pahio (2872) pahio (2872) 5 pahio (2872) Theorem msc 13A05 msc 13A18 msc 12J20 TwoGeneratorProperty SumOfIdeals