CatDat

regular subobject classifier

A category $\mathcal{C}$ has a regular subobject classifier if it has finite limits and a regular monomorphism* $\top : 1 \to \Omega$ such that for every regular monomorphism $m : A \to B$ there is a unique morphism $\chi_m : B \to \Omega$ such that $B \leftarrow A \rightarrow 1$ is the pullback of $B \rightarrow \Omega \leftarrow 1$. Equivalently, the functor $\mathrm{Sub}_{\mathrm{reg}} : \mathcal{C}^{\mathrm{op}} \to \mathbf{Set}^+$ is representable.
*Every morphism $1 \to \Omega$ is a split monomorphism and hence regular anyway.

• Related properties: finitely complete, subobject classifier
• nLab Link

Relevant implications

• additive andregular subobject classifier implies trivial
• finitely complete andright cancellative implies regular subobject classifier
• mono-regular andregular subobject classifier implies subobject classifier
• regular subobject classifier andstrict terminal object implies thin
• regular subobject classifier implies finitely complete
• subobject classifier implies regular subobject classifier

Examples

There are 19 categories with this property.

• category of combinatorial species
• category of finite sets
• category of M-sets
• category of measurable spaces
• category of pairs of sets
• category of pointed sets
• category of prosets
• category of sets
• category of sheaves
• category of simplicial sets
• category of topological spaces
• poset [0,1]
• poset of extended natural numbers
• proset of integers w.r.t. divisibility
• trivial category
• walking commutative square
• walking composable pair
• walking isomorphism
• walking morphism

Counterexamples

There are 39 categories without this property.

• category of abelian groups
• category of abelian sheaves
• category of algebras
• category of Banach spaces with linear contractions
• category of commutative algebras
• category of commutative monoids
• category of commutative rings
• category of fields
• category of finite abelian groups
• category of finite orders
• category of finite sets and bijections
• category of finite sets and injections
• category of finite sets and surjections
• category of finitely generated abelian groups
• category of free abelian groups
• category of groups
• category of Hausdorff spaces
• category of left modules over a division ring
• category of left modules over a ring
• category of metric spaces with continuous maps
• category of monoids
• category of non-empty sets
• category of posets
• category of rings
• category of rngs
• category of sets and relations
• category of smooth manifolds
• category of vector spaces
• delooping of a non-trivial finite group
• delooping of an infinite group
• delooping of the additive monoid of natural numbers
• delooping of the additive monoid of ordinal numbers
• discrete category on two objects
• empty category
• poset of natural numbers
• poset of ordinal numbers
• walking fork
• walking parallel pair of morphisms
• walking span

Unknown

There are 7 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 metric spaces with non-expansive maps
• category of metric spaces with ∞ allowed
• category of schemes
• category of small categories
• category of Z-functors
• dual of the category of sets