Let $Q$ be a finite acyclic quiver, and $X$ some representation of $Q$. For $i \in Q\_0$ define the $kQ$-modules $P\_i = kQe\_i$, and $X(i) = e\_i X$. The representation $X$ has a canonical projective resolution, called the Ringel resolution, given by 
$$
0 
\longrightarrow 
\bigoplus\_{a \in Q\_1} P\_{h(a)} \otimes X(t(a))
\xrightarrow{\quad d \quad}
\bigoplus\_{i \in Q\_0} P\_i \otimes X(i)
\xrightarrow{\quad f \quad}
X
\longrightarrow 
0
$$
where in each direct summand the maps are given by 
$$
d(p \otimes x) = p \otimes (a \cdot x) - pa \otimes x
\qquad\quad
f(p \otimes x) = p \cdot x
$$


Does anyone have some intuition for this resolution that they could share? Can the modules and the maps in this resolution be nicely interpreted in terms of the paths in $Q$? And can this interpretation be extended to get us projective resolutions for quivers with relations? Or for non-acylic quivers?



Intuition behind the canonical projective resolution of a quiver representation
I suggest to think about it in terms of an analogy. If $G$ is a group, and $M$ is a module over $G$, you have a resolution of $M$ known as the bar complex. The way you produce this resolution can be thought of as follows. You freely generate a module over $G$ by elements of $M$, namely consider $\mathbb{Z}[G] \otimes M$, with the action only on the first coordinate. This maps to $M$ by $g \otimes m \mapsto gm$. Then, there are many relations, and the only uniform way to encode all of them in general is to note that the defining relation for a module is $g(hm) = (gh)m$ so you encode this relation using $\mathbb{Z}[G \times G] \otimes M$. Then essentially the only way which is uniform in $M$ and $G$ to encode the relation among those is to use $G \times G \times G$, e.t.c. 


In general, if $C$ is a category, you can consider modules over $C$, and to construct a very similar resolution, encoding the full uniform collection of relations you can impose on a module without any assumptions on $C$ or the module $M$. The resulting complex is the bar construction of $C$ with coefficients in $M$, and a typical element in it is a tuple of composable maps, tensored with a module element concentrated at the source of the first map. The details are not important for our discussion, just that it is similar to the bar resolution for group cohomology. 


Now, if $G$ turn out to be a $\textbf{free}$ group, you can write a much shorter resolution for $M$ over $G$. Namely, choose a set of generators $S$ for $G$ and then you can resolve $M$ by 
$$\mathbb{Z}[G]^S\otimes M \to \mathbb{Z}[G] \otimes M \to M$$
where the first map is $g \otimes m \mapsto gm$ and the second is 
$(x)\_i \otimes m \mapsto xs\_i \otimes m - x \otimes s\_im$. Again we implement the relations defining $M$ as a module, but this time we restrict only to the relations on the free generators. The fact that they generate $G$ imply that all relations follows from these, and the fact that they freely generate $G$ imply that there are no relations-among-relations, so we get a very short resolution. 


As we saw, a group can be generalized to a category, and the analogous notion of a free group is a quiver: A category freely generated by some objects and morphisms. The resolution you asked about is the analogous resolution to the one I discussed for the free group. You write down only the "module relations" coming from the generating morphisms, and the fact that they freely generate the whole collection of morphisms, imply that there are no relations-among-relations, so we get a very short resolution. 


