The notion of a normal subobject is the proper generalization of a normal subgroup to other algebraic categories. The notion was found relatively late. In the category of groups, there are two equivalent descriptions of a normal subgroup: as the kernel of a homomorphism of groups and as the equivalence class of the unit of some (necessarily unique) congruence.

Given a category $C$ admitting finite limits, one says that a morphism $f:X\to Y$ in $C$ is **normal** to the internal equivalence relation $r: R\hookrightarrow Y\times Y$ if it factors through the monomorphism $r$ (i.e. $\exists \bar{f}$ such that $r\circ\bar{f}=f$ and …)

A **normal subobject** is a monomorphism which is normal to some equivalence relation.

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A protomodular category is defined in such a way that it possesses an intrinsic notion of normal subobject.

See also normal monomorphism

Last revised on August 16, 2016 at 14:48:53. See the history of this page for a list of all contributions to it.