View source for Projective covering

''of a left module $M$ over a ring $R$''

The dual notion to that of an [[injective envelope]] or injective hull. Let $R$ be an associative ring with unit element, $M$ a left module over $R$. From now on, all modules and morphisms are left modules and morphisms of left modules. An epimorphism $q : P \rightarrow M$ is an ''essential'' epimorphism if the following holds: $u :P \rightarrow P'$ is an epimorphism if and only if $qu$ is an epimorphism. This is equivalent to saying that $\ker q$ is a [[superfluous submodule]], where $N \subseteq M$ is ''superfluous'' if for all submodules $M' \subseteq M$ one has: $M' + N = M$ implies $M' = M$. The notion of an essential epimorphism is dual to that of an essential monomorphism (or essential extension), which is a monomorphism $j : M \rightarrow Q$ such that $v : M' \rightarrow M$ is monomorphic if and only if $jv$ is monomorphic: this is equivalent to the condition that $\mathrm{im}\, v$ is an [[essential submodule]]. A projective covering of $M$ is a [[projective module]] $P$ together with an essential epimorphism $q : P \rightarrow M$. In contrast to the dual notion of an injective envelope (an [[injective module]] $Q$ together with an essential monomorphism $M \rightarrow Q$) projective coverings do not always exist. For instance, indeed especially, projective coverings of Abelian groups ($\mathbf{Z}$-modules) do not exist. The rings for which projective coverings of modules do exist have been characterized [[#References|[a1]]] (cf. also [[Perfect ring]]).

These notions are completely categorical. A [[Grothendieck category]] (also called an $AB5$ category with [[Generator of a category|generator]]s) has the property that injective envelopes always exist.

====References====
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<TR><TD valign="top">[a1]</TD> <TD valign="top">  H. Bass,   "Finitistic homological dimension and a homological generalization of semi-primary rings"  ''Trans. Amer. Math. Soc.'' , '''95'''  (1960)  pp. 466–488</TD></TR>
<TR><TD valign="top">[a2]</TD> <TD valign="top">  N. Popescu,   "Abelian categories with applications to rings and modules" , Acad. Press  (1973)  pp. Sect. 3.10</TD></TR>
<TR><TD valign="top">[b1]</TD> <TD valign="top">  F.W. Anderson, K.R. Fuller, "Rings and Categories of Modules" Graduate Texts in Mathematics '''13''' Springer (2012) {{ISBN|1468499130}}</TD></TR>
</table>

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