Subobject

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,[1] 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.

The dual concept to a subobject is a quotient object. This generalizes concepts such as quotient sets, quotient groups, quotient spaces, quotient graphs, etc.

## Definitions

In detail, let A be an object of some category. Given two monomorphisms

u : S -> A and v : T -> A

with codomain A, we write u v if u factors through v--that is, if there exists ? : S -> T such that ${\displaystyle u=v\circ \varphi }$. The binary relation ? defined by

u ? v if and only if u v and v u

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 ${\displaystyle u=v\circ \varphi }$.)

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.

## Examples

1. In Set, the category of sets, a subobject of A corresponds to a subset B of A, or rather the collection of all maps from sets equipotent to B with image exactly B. The subobject partial order of a set in Set is just its subset lattice.
2. In Grp, the category of groups, the subobjects of A correspond to the subgroups of A.
3. Given a partially ordered class P, we can form a category with P's elements as objects and a single arrow going from one object (element) to another if the first is less than or equal to the second. If P has a greatest element, the subobject partial order of this greatest element will be P itself. This is in part because all arrows in such a category will be monomorphisms.
4. A subobject of a terminal object is called a subterminal object.

## Notes

1. ^ Mac Lane, p. 126

## References

• Mac Lane, Saunders (1998), Categories for the Working Mathematician, Graduate Texts in Mathematics, 5 (2nd ed.), New York, NY: Springer-Verlag, ISBN 0-387-98403-8, Zbl 0906.18001
• Pedicchio, Maria Cristina; Tholen, Walter, eds. (2004). Categorical foundations. Special topics in order, topology, algebra, and sheaf theory. Encyclopedia of Mathematics and Its Applications. 97. Cambridge: Cambridge University Press. ISBN 0-521-83414-7. Zbl 1034.18001.

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