# Subquotient Functorial Construction¶

AUTHORS:

• Nicolas M. Thiery (2010): initial revision
sage.categories.subquotients.Subquotients(self)
INPUT:
• self – a concrete category

Given a concrete category self == As() (i.e. a subcategory of Sets()), As().Subquotients() returns the category of objects of As() endowed with a distinguished description as subquotient of some other object of As().

EXAMPLES:

sage: Monoids().Subquotients()
Category of subquotients of monoids


A parent $$A$$ in As() is further in As().Subquotients() if there is a distinguished parent $$B$$ in As(), called the ambient space, a subspace $$B'$$ of $$B$$ and a pair of structure preserving maps:

$l: A \mapsto B' \text{ and } r: B' \mapsto A$

called respectively the lifting map and retract map such that $$r \circ l$$ is the identity of $$A$$. What exactly structure preserving means is explicited in each category; this typically states that, for each operation $$op$$ of the category, there is a commutative diagram such that:

for all $$e\in A$$, one has $$op_A(e) = r(op_B(l(e)))$$

This allows for deriving the operations on $$A$$ from those on $$B$$.

Note: this is a slightly weaker definition than that found on http://en.wikipedia.org/wiki/Subquotient: B’ is not necessarily required to be a subobject of B.

Assumptions:

• For any category As(), As().Subquotients() is a subcategory of As().

Example: a subquotient of a group is a group (e.g. a left or right quotients of a group by a non normal subgroup is not in this category).

• This construction is covariant: if As() is a subcategory of Bs(), then As().Subquotients() is a subcategory of Bs().Subquotients()

Example: if $$A$$ is a distinguished subquotient of $$B$$ in the category of groups, then is is also a subquotient of $$B$$ in the category of monoids.

• If the user (or a program) calls As().Subquotients(), then it is assumed that subquotients are well defined in this category. This is not checked, and probably never will. Note that, if a category $$As()$$ does not specify anything about its subquotients, then it’s subquotient category looks like this:

sage: EuclideanDomains().Subquotients()
Join of Category of euclidean domains and Category of subquotients of monoids


Interface: the ambient space of $$B$$ is given by B.ambient(). The lifting and retract map are implemented respectively as methods B.lift(b) and B.retract(a). As a shorthand, one can use alternatively b.lift():

sage: S = Semigroups().Subquotients().example(); S
An example of a (sub)quotient semigroup: a quotient of the left zero semigroup
sage: S.ambient()
An example of a semigroup: the left zero semigroup
sage: S(3).lift().parent()
An example of a semigroup: the left zero semigroup
sage: S(3) * S(1) == S.retract( S(3).lift() * S(1).lift() )
True


See S? for more.

TODO: use a more interesting example, like $$\ZZ/n\ZZ$$.

The two most common use cases are:

• quotients, when $$A'=A$$ and $$r$$ is a morphism; then $$r$$ is a canonical quotient map from $$A$$ to $$B$$)
• subobjects (when $$l$$ is an embedding from $$B$$ into $$A$$).

See respectively Quotients and Subobjects.

TESTS:

sage: TestSuite(Sets().Subquotients()).run()

class sage.categories.subquotients.SubquotientsCategory(category, *args)

TESTS:

sage: from sage.categories.covariant_functorial_construction import CovariantConstructionCategory
sage: class FooBars(CovariantConstructionCategory):
...       _functor_category = "FooBars"
sage: Category.FooBars = lambda self: FooBars.category_of(self)
sage: C = FooBars(ModulesWithBasis(ZZ))
sage: C
Category of foo bars of modules with basis over Integer Ring
sage: C.base_category()
Category of modules with basis over Integer Ring
sage: latex(C)
\mathbf{FooBars}(\mathbf{ModulesWithBasis}_{\Bold{Z}})
sage: import __main__; __main__.FooBars = FooBars # Fake FooBars being defined in a python module
sage: TestSuite(C).run()


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