**Question 1:** Let $\mathcal A$ be an abelian group. Does there exist an inverse system $(A^n)\_{n \in \mathbb N} = (\cdots \to A^n \to A^{n-1} \to \cdots \to A^0)$ such that $\varprojlim^1 A^\bullet \cong \mathcal A$? If not, can we characterize the abelian groups which are $\varprojlim^1$ groups or at least say anything interesting about their isomorphism types?


The remaining questions are meant to be refinements of Question 1.


**Question 2:** Let $\mathcal B^0,\mathcal B^1$ be abelian groups. Does there exist an inverse system $(B^n)\_{n \in \mathbb N}$ such that $\varprojlim^i B^\bullet \cong \mathcal B^i$ for $i=0,1$?


If $(C^n)\_{n \in \mathbb N}$ is an inverse system, there is a canonical two-term chain complex which we'll call $\mathbf{Lim} (C^\bullet) = (\prod\_{n \in \mathbb N} C^n \to \prod\_{n \in \mathbb N} C^n)$, where the differential is $(c^0,c^1,\dots) \mapsto (c^0 - \gamma c^1,c^1-\gamma c^2,\dots)$ where $\gamma$ ambiguously denotes any of the linking maps for the inverse system $C^\bullet$. The point, of course, is that $H^i(\mathbf{Lim} (C^\bullet)) = \varprojlim^i(C^\bullet)$ for $i=0,1$.


**Question 3:** Let $\mathcal C^\ast = (\mathcal C^0 \to \mathcal C^1)$ be a two-term chain complex of abelian groups. Does there exist an inverse system $(C^n)\_{n \in \mathbb N}$ of abelian groups such that $\mathbf{Lim}(C^\bullet)$ is quasi-isomorphic to $\mathcal C^\ast$?


If $(D^{n,\ast})\_{n \in \mathbb N}$ is an inverse system of chain complexes of abelian groups, then define $\mathbf{Lim}(D^{\bullet,\ast})$ by by applying $\mathbf{Lim}$ levelwise to obtain a double complex, and then taking the diagonal.


**Question 4:** Let $\mathcal D^\ast$ be a chain complex of abelian groups. Does there exist an inverse system $(D^{n,\ast})\_{n \in \mathbb N}$ of chain complexes of abelian groups such that $\mathbf{Lim}(D^{\bullet,\ast})$ is quasi-isomorphic to $\mathcal D^\ast$?



Which abelian groups are $\varprojlim^1$ groups?
Abelian group $A$ is cotorsion if $\rm{Ext}(F, A) = 0$ for every flat $F$, or, equivalently, $\rm{Ext}(\Bbb Q, A) = 0$


Every $\varprojlim^1$ of an inverse system of abelian group is cotorsion, and, conversely, every cotorsion group is a $\varprojlim^1$. Proof can be found in *Warfield, Huber. On the values of the functor $\varprojlim^1$*.


If every group in the inverse system $G\_i$ is f. g., then $\varprojlim^1$ is $\rm{Ext}(A, \Bbb Z)$ for $A$ torsion free countable: take $A$ equal to direct limit of $\rm{Hom}(G\_i, \Bbb Z)$. In particular, that limit is always a divisible group.


Converse is also true, but version of proof I know is messy and uses structure teorem of alg. compact groups. Probably you can find something on that in Fuchs or Jensen books.


