Projective length, phantom extensions, and the structure of flat modules

We consider the natural generalization of the notion of the order of a phantom map from the topological setting to triangulated categories. When applied to the derived category of the category of countable flat modules over a countable Dedekind domain, this yields a notion of\emph{\ phantom extension} of order $\alpha <\omega _{1}$. We provide a complexity-theoretic characterization of the module $\mathrm{Ph}$ of phantom extensions of order $\alpha$ with respect to the structure of \emph{phantom Polish module} on $\mathrm{Ext}\left( C,A\right)$ obtained by considering it as an object of the left heart of the quasi-abelian category of Polish modules. We use this characterization to prove the following Dichotomy Theorem: either all the extensions of a countable flat module $A$ are trivial (which happens precisely when $A$ is divisible) or $A$ has phantom extensions of arbitrarily high order. By producing canonical phantom projective resolutions of order $\alpha$, we prove that phantom extensions of order $\alpha$ define on the category of countable flat modules an exact structure $\mathcal{E}_{\alpha }$ that is hereditary with enough projectives, and the functor $\mathrm{Ph}^{\alpha }$ is the derived functor of $\mathrm{Hom}$ with respect to $\mathcal{E}_{\alpha }$. We prove a Structure Theorem characterizing the objects of the class $\mathcal{P}_{\alpha }$ of countable flat modules that have \emph{projective length at most }$\alpha$ (i.e., are $\mathcal{E}_{\alpha }$-projective) as the direct summands of colimits of presheaves of finite flat modules over well-founded forests of rank $$ regarded as ordered sets. This can be seen as the first analogue in the flat case of the classical Ulm Classification Theorem for torsion modules.