essentially small

A category is essentially small when it is equivalent to a small category. Equivalently, it is locally essentially small and its collection of isomorphism classes of objects is essentially small (isomorphic to a set). See here for the set-theoretic foundation of category theory we are working with. In contrast to the property of being small, being essentially small is invariant under equivalences of categories.

Relevant implications

• accessible andessentially discrete implies essentially small
• Cauchy complete andessentially small implies accessible
• Cauchy complete andessentially small implies coaccessible
• coaccessible andessentially discrete implies essentially small
• cocomplete andessentially small andinfinitary codistributive andthin implies cocartesian coclosed
• cocomplete andessentially small andthin implies complete
• complete andessentially small andinfinitary distributive andthin implies cartesian closed
• complete andessentially small andthin implies cocomplete
• copowers andessentially small implies thin
• essentially countable implies essentially small
• essentially discrete andmulti-initial object implies essentially small
• essentially discrete andmulti-terminal object implies essentially small
• essentially small andgroupoid implies finitely accessible andgeneralized variety
• essentially small andpowers implies thin
• essentially small implies extremal cogenerating collection andlocally essentially small andwell-copowered andwell-powered
• essentially small implies extremal generating collection andlocally essentially small andwell-copowered andwell-powered
• extremal cogenerating collection andlocally finite andright cancellative andsemi-strongly connected implies essentially small
• extremal generating collection andleft cancellative andlocally finite andsemi-strongly connected implies essentially small
• locally copresentable andlocally presentable implies essentially small
• small implies essentially small

Examples

There are 49 categories with this property.

• category of combinatorial species
• category of countable groups
• category of countable sets
• category of countable-dimensional vector spaces
• category of finite abelian groups
• category of finite groups
• category of finite ordered sets
• category of finite 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 Z-sets
• 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 smooth manifolds
• 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
• discrete category on two objects
• empty category
• forked commutative square
• indiscrete category of sets
• partially ordered set of extended natural numbers
• partially ordered set of natural numbers
• preordered set of integers w.r.t. divisibility
• real interval [0,1]
• simplex category
• trivial category
• walking commutative square
• walking composable pair
• walking coreflexive pair
• walking fork
• walking idempotent
• walking isomorphism
• walking morphism
• walking parallel pair
• walking span
• walking splitting

Counterexamples

There are 84 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 connected sequences of sets
• category of coproducts of Euclidean spaces
• category of empty-or-finite pairs of sets
• category of F(I)-sets
• category of fields
• category of fields of characteristic zero
• category of filtered vector spaces
• 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 Jónsson-Tarski algebras
• category of large families of abelian groups
• category of large families of sets
• 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 locally ringed spaces
• category of long transfinite sequences of abelian groups
• category of M-sets
• category of measurable spaces
• 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 pairs of sets
• category of partially ordered sets
• category of partially ordered sets without isolated points
• 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 rings
• category of rngs
• category of schemes
• category of semigroups
• category of sequences of abelian groups
• category of sequences of sets
• category of set functions and commutative squares
• category of sets
• category of sets and bijections
• category of sets and relations
• 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 irreflexive binary relation
• category of sets equipped with a symmetric reflexive binary relation
• category of sets equipped with an irreflexive binary relation
• category of sets with a distinguished subset
• category of sets with finite-to-one maps
• category of sheaves
• category of simplicial sets
• category of small categories
• category of topological spaces
• category of torsion abelian groups
• category of torsion-free abelian groups
• category of uniform spaces
• category of vector spaces
• category of Z-functors
• category of Z-sets
• cocompletion of a discrete–pair join
• delooping of the additive monoid of ordinal numbers
• discrete category of sets
• dual of the category of sets
• dual of the category of topological spaces
• partially ordered collection of ordinal numbers

Unknown

There are 0 categories for which the database has no information on whether they satisfy this property.

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