In category theory, a branch of mathematics, a subobject is, roughly speaking, an object that sits inside another object in the same category. The notion is a generalization of concepts such as subsets from set theory, subgroups from group theory, and subspaces from topology. Since the detailed structure of objects is immaterial in category theory, the definition of subobject relies on a morphism that describes how one object sits inside another, rather than relying on the use of elements.
In detail, let A be an object of some category. Given two monomorphisms
is an equivalence relation on the monomorphisms with codomain A, and the corresponding equivalence classes of these monomorphisms are the subobjects of A. (Equivalently, one can define the equivalence relation by u ? v if and only if there exists an isomorphism ? : S -> T with .)
The relation partial order on the collection of subobjects of A.
The collection of subobjects of an object may in fact be a proper class; this means that the discussion given is somewhat loose. If the subobject-collection of every object is a set, the category is called well-powered or sometimes locally small.
To get the dual concept of quotient object, replace monomorphism by epimorphism above and reverse arrows. A quotient object of A is then an equivalence class of epimorphisms with domain A.
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