There is a common characterization of the class group ${\rm Cl}(R)$ as a kind of measure of how badly factorization fails to be unique. The most obvious justification for this sentiment is that the order of the class group of $R$ determines if it has unique factorization or not:
$$h=1\quad\Leftrightarrow\quad {\rm PID}\quad \Rightarrow \quad {\rm UFD}. \tag{$\star$}$$
This is very unsatisfying though because the exact size of $h$ (not even to mention all of the group structure that ${\rm Cl}(R)$ has in general) is not used, and "UFD / not UFD" doesn't in any sense measure the extent to which $R$ fails to be a UFD, only if it does. Plus, the converse, UFD $\Rightarrow$ PID, is specific to when $R$ is Dedekind, so this is only a partial justification.
There are a number of answers exposing specific ideas in How does a class group measure the failure of unique factorization? (which is basically my question here, but I am resurrecting it because I am not satisfied) and Class number measuring failure of unique factorization at MO.
PLC and BD cite Carlitz ($h=2$ iff factorization lengths are invariant), PLC notes that $h$ can yield arithmetic obstructions to certain paths like in FLT, and BD cites a theorem of Kaczorowski which goes from factorization information to an exact isomorphism class characterization of the class group (which is reverse of the desired order: going from class group to factorization information).
Kevin has the strongest skeptical vibe among the responses in his conclusion:
Unfortunately, you can see we lose a lot of information in passing to the class group, and in particular it doesn't tell us anything at all about which elements are obstacles to unique factorization. The intuition, rather, is that a more complicated class group implies we're further from unique factorization.
I am not sure how the mentioned intuition translates into concrete facts, but I do get the impression that $\rm Cl$ doesn't have the right "type" of information to talk about factorizations. Rather, I think the more direct description for $\rm Cl$ is as a measure of how ideals fail to act like numbers.
Here is my own crack at "measuring" factorization's failure to be unique. Let $\Gamma(R)$ be the set of all associates classes of irreducible elements. (Suppose $R$ is a factorization domain, so all elements have some factorization if not a unique one, and $K$ is $R$'s fraction field.) The group of principal fractional ideals is essentially $K^\times/R^\times$, and it is generated by irreducible elements. Thus there is a surjective map $\Bbb Z^{\oplus \Gamma(R)}\to K^\times/R^\times$, and the kernel is comprised of all relations satisfied between irreducibles under multiplication. These relations are precisely the inequivalent factorizations into irreducibles that occur in $R$. Our knowledge so far can be put into a diagram:
$$\{{\rm relations}\}\to\Bbb Z^{\oplus \Gamma(R)}\to K^\times/R^\times\to I(R)\to{\rm Cl}(R).$$
Observe this has two subsequences which are short exact, the first ending and the second beginning with $K^\times/R^\times$. (Can we determine if $\rm relations$ is infinitely generated or not?) One thing to notice is that in such a sequence $A\to B\to C\to D\to E$, the exact sequence $C\to D\to E$ generically is expected to have little control over the exact sequence $A\to B\to C$, which at face value seems like good evidence that ${\rm Cl}(R)$ does not influence factorization very much, but in fact $(\star)$ says $|{\rm Cl}|=1$ implies $\{\rm relations\}=0$, so this diagram does not fully capture the situation.
So, my questions:
- What direct, specific relationships exist between elemental factorizations and ideal classes? (In particular, beyond Kaczorowski's theorem.)
- Why is it useful to think of ${\rm Cl}(R)$ as measuring the failure of unique factorization, as opposed to being only indirectly related (i.e. by measuring the failure of ideals to act like numbers)?
- What properties of ${\rm Cl}(R)$ definitively don't say anything specific about factorizations, and conversely what properties of factorizations are definitively not captured by ideal classes?
Sorry if I have been rambling, or not justified posting about this again, or my questions too vague.