# nLab superfluous epimorphism

Given a ring $R$ (or some analogue, say a Banach algebra), a submodule $K$ of an $R$-module $M$ is called superfluous or small in $M$, written $K \ll M$, if, for every submodule $L\subset M$ , the equality $K + L = M$ implies $L = M$. An epimorphism $f : M\to N$ is called superfluous (or coessential) if $Ker f \ll M$.

Superfluous epimorphisms (submodules) are a notion dual to essential monomorphisms (submodules); their role in the study of projective covers is analogous to the role of essential monomorphisms in the study of injective envelopes.

category: algebra

Last revised on January 14, 2018 at 20:18:45. See the history of this page for a list of all contributions to it.