Andre Weil noticed as a student in 1925 that the polynomial ring $\mathbb{Z}[x]$ comes close to being a PID, and he describes this as `` the embryo of my future thesis.'' 


He observed that, given $f(x),g(x)\in\mathbb{Z}[x]$, the Euclidean algorithm computes a sequence of polynomials where each is a linear combination of the preceding ones and which either:


1) Ends in a term which divides the preceding term and thus is something like a GCD for $f$ and $g$; or


2) Ends in an integer $d$ which does not divide the preceding term but bounds the common divisors of values pointwise in this way: For any integer $n$, an integer common divisor of $f(n),g(n)$ must divide $d$.


I have not seen this property discussed anywhere. Does it have a name? 


There is related discussion in the question [The resultant and the ideal generated by two polynomials in $\mathbb{Z}[x]$](https://mathoverflow.net/questions/17501/the-resultant-and-the-ideal-generated-by-two-polynomials-in-mathbbzx?noredirect=1&lq=1)


The number $d$ is called the reduced resultant of $f,g$. 



Is there a name for this property Weil saw for integer polynomials?
I do not know of a name. But Weil's observation follows from properties of the resultant, and generalizes to other rings $A[x]$ ($A$ a GCD domain).


The resultant $R(f,g)$ of two polynomials is defined either as a certain determinant or as a certain product over pairs of roots of $f,g$, see [this Wikipedia page](https://en.wikipedia.org/wiki/Resultant).


From the two definitions, we see that:


* The resultant vanishes iff $f,g$ have common root (this is basically the definition of the resultant).
* The resultant has the property that there exist $p,q \in \mathbb{Z}[x]$ ($\deg p < \deg g$, $\deg q < \deg f$) such that $$(\*) p(x)f(x)+q(x)g(x)=R(f,g)$$ identically. In particular, $R(f,g) \in (f(x),g(x))$ - this ideal contains a non-zero integer!


So,


* If the resultant is 0, there is a genuine GCD.
* Otherwise, we end up with a non-zero integer $R(f,g)$. Plugging $x=n$ in $(\*)$ we see that indeed the GCD of $f(n),g(n)$ divides $R(f,g)$.


The above observations generalize to $A[x]$ when $A$ has the GCD property. 


As explained in the wikipedia page, the Euclidean algorithm differs from the resultant calculation by a simple factor.




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It is interesting to note that the resultant $R(f,g)$ is not the necessarily the generator of $(f(x),g(x)) \cap \mathbb{Z}$, which we will denote $c(f,g)$. [Myerson has shown](http://www.ams.org/journals/proc/1983-089-03/S0002-9939-1983-0715856-2/home.html) that $R(f,g)$ divides an (effective) power of $c(f,g)$.
When $R(f,g)$ is squarefree and $f,g$ are monics it may be shown that $c(f,g)=R(f,g)$. This may be seen from the results of this [recent paper of Frenkel and Pelikán](https://arxiv.org/abs/1608.07936), who study the possibly values of $\gcd(f(n),g(n))$ as $n$ ranges over $\mathbb{Z}$.


