regular subobject classifier

A category $\C$ has a regular subobject classifier if it has finite limits and a regular monomorphism* $\top : 1 \hookrightarrow \Omega$ such that for every regular monomorphism $m : A \hookrightarrow B$ there is a unique morphism $\chi_m : B \to \Omega$ such that $$\begin{CD} A @>{m}>> B \\ @V{!}VV @VV{\chi_m}V \\ 1 @>>{\top}> \Omega \end{CD}$$ is a pullback diagram. Equivalently, the functor $\Sub_{\reg} : \C^{\op} \to \Set^+$ is representable. *Every morphism $1 \to \Omega$ is a split monomorphism and hence regular anyway.

Relevant implications

• additive andregular subobject classifier implies trivial
• finitely complete andright cancellative implies regular subobject classifier
• Malcev andregular subobject classifier implies regular-subobject-trivial
• mono-regular andregular subobject classifier implies subobject classifier
• quasitopos is equivalent to finitely cocomplete andfinitely complete andlocally cartesian closed andregular subobject classifier
• regular quotient object classifier andself-dual implies regular subobject classifier
• regular subobject classifier andself-dual implies regular quotient object classifier
• regular subobject classifier andstrict terminal object implies thin
• regular subobject classifier implies finitely complete
• subobject classifier implies regular subobject classifier

Examples

There are 40 categories with this property.

• category of combinatorial species
• category of connected sequences of sets
• category of countable sets
• category of empty-or-finite pairs of sets
• category of F(I)-sets
• category of finite sets
• category of finite Z-sets
• category of Jónsson-Tarski algebras
• category of large families of sets
• category of M-sets
• category of measurable spaces
• category of pairs of sets
• category of pointed sets
• category of pointed topological spaces
• category of preordered sets
• category of pseudo-metric spaces with non-expansive maps
• category of quivers
• category of quivers with finite components
• category of sequences of sets
• category of set functions and commutative squares
• category of sets
• category of sets equipped with a binary relation
• category of sets equipped with a reflexive binary relation
• category of sets equipped with a symmetric binary relation
• category of sets equipped with a symmetric reflexive binary relation
• category of sets with a distinguished subset
• category of sheaves
• category of simplicial sets
• category of topological spaces
• category of uniform spaces
• category of Z-sets
• indiscrete category of sets
• partially ordered set of extended natural numbers
• preordered set of integers w.r.t. divisibility
• real interval [0,1]
• trivial category
• walking commutative square
• walking composable pair
• walking isomorphism
• walking morphism

Counterexamples

There are 90 categories without this property.

• category of abelian groups
• category of abelian sheaves
• category of algebras
• category of Banach spaces with linear contractions
• category of cochain complexes of abelian groups
• category of commutative algebras
• category of commutative monoids
• category of commutative rings
• category of compact Hausdorff spaces
• category of coproducts of Euclidean spaces
• category of countable groups
• category of countable-dimensional vector spaces
• category of fields
• category of fields of characteristic zero
• category of filtered vector spaces
• category of finite abelian groups
• category of finite groups
• category of finite ordered sets
• category of finite sets and bijections
• category of finite sets and injections
• category of finite sets and surjections
• category of finite sets of cardinality a power of 3
• category of finite sets of even cardinality
• category of finite sets of odd cardinality
• category of finite-dimensional vector spaces
• category of finite-dimensional vector spaces [countable field]
• category of finite-dimensional vector spaces [finite field]
• category of finite-dimensional vector spaces [uncountable field]
• category of finitely generated abelian groups
• category of finitely generated free abelian groups
• category of finitely generated free modules over Z x Z
• category of finitely generated projective modules over the ring of dual numbers
• category of free abelian groups
• category of graded abelian groups
• category of graded modules over a graded ring
• category of groups
• category of Hausdorff spaces
• category of large families of abelian groups
• category of large families of sets and bijections
• category of large families of sets which are mostly empty
• category of large families of sets which are mostly singletons
• category of large families of vector spaces which are mostly zero
• category of large vector spaces over a large field with a small basis
• category of left modules over a division ring
• category of left modules over a non-semisimple ring
• category of left modules over a ring
• category of long transfinite sequences of abelian groups
• category of metric spaces with continuous maps
• category of metric spaces with non-expansive maps
• category of metric spaces with ∞ allowed
• category of monoids
• category of non-empty sets
• category of partially ordered sets
• category of partially ordered sets without isolated points
• category of rings
• category of rngs
• category of semigroups
• category of sequences of abelian groups
• category of sets and bijections
• category of sets and relations
• category of sets equipped with a symmetric irreflexive binary relation
• category of sets equipped with an irreflexive binary relation
• category of sets with finite-to-one maps
• category of small categories
• category of smooth manifolds
• category of torsion abelian groups
• category of torsion-free abelian groups
• category of vector spaces
• cocompletion of a discrete–pair join
• delooping of a group
• delooping of a non-trivial finite group
• delooping of an infinite countable group
• delooping of an infinite uncountable group
• delooping of the additive monoid of natural numbers
• delooping of the additive monoid of ordinal numbers
• discrete category of sets
• discrete category on two objects
• dual of the category of sets
• dual of the category of topological spaces
• empty category
• forked commutative square
• partially ordered collection of ordinal numbers
• partially ordered set of natural numbers
• simplex category
• walking coreflexive pair
• walking fork
• walking idempotent
• walking parallel pair
• walking span
• walking splitting

Unknown

There are 3 categories for which the database has no information on whether they satisfy this property. Please help us fill in the gaps by contributing to this project.

• category of locally ringed spaces
• category of schemes
• category of Z-functors