CatDat

extremal generator

An object $G$ of a category is called an extremal generator if it is a generator and for every morphism $f : A \to B$, if $f\circ{-} : \Hom(G,A)\to\Hom(G,B)$ is a bijection, then $f$ is an isomorphism. Equivalently, the functor $\Hom(G,-) : \C \to \Set^+$ is faithful and conservative. This property refers to the existence of an extremal generator.
In a locally essentially small category with small coproducts, it is also equivalent to the condition that the canonical morphism $$\textstyle\bigsqcup_{f\in\Hom(G,A)} G \to A$$ is an extremal epimorphism for every object $A$, explaining the terminology (see Prop. 5.3 at the nLab).
By definition, $G$ is an extremal generator if and only if $\{G\}$ is an extremal generating set.

• Dual property: extremal cogenerator
• Related properties: extremal generating set, generator
• nLab Link

Relevant implications

• balanced andgenerator implies extremal generator
• coproducts andextremal generating set andstrongly connected implies extremal generator
• core-connected implies extremal generator
• extremal cogenerator andself-dual implies extremal generator
• extremal generator andself-dual implies extremal cogenerator
• extremal generator implies extremal generating set andgenerator
• finitary algebraic implies extremal generator
• generator andgroupoid implies extremal generator

Examples

There are 62 categories with 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 compact Hausdorff spaces
• category of countable groups
• category of countable sets
• category of filtered vector spaces
• category of finite ordered sets
• category of finite sets
• category of finite sets and injections
• 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 free abelian groups
• category of groups
• category of Jónsson-Tarski algebras
• category of left modules over a division ring
• category of left modules over a ring
• category of M-sets
• 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 pointed sets
• category of preordered sets
• category of pseudo-metric spaces with non-expansive maps
• category of rings
• category of rngs
• category of semigroups
• category of sets
• category of sets and relations
• category of sets with finite-to-one maps
• category of simplicial sets
• category of small categories
• category of smooth manifolds
• category of torsion abelian groups
• category of torsion-free abelian groups
• category of vector spaces
• 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 on two objects
• dual of the category of sets
• dual of the category of topological spaces
• simplex category
• trivial category
• walking coreflexive pair
• walking fork
• walking idempotent
• walking isomorphism
• walking morphism
• walking parallel pair
• walking splitting

Counterexamples

There are 25 categories without this property.

• category of combinatorial species
• category of fields
• category of finite abelian groups
• category of finite groups
• category of finite sets and bijections
• category of finite sets and surjections
• category of Hausdorff spaces
• category of locally ringed spaces
• category of measurable spaces
• category of pairs of sets
• category of pointed topological spaces
• category of schemes
• category of set functions and commutative squares
• category of sets with a distinguished subset
• category of sheaves
• category of topological spaces
• empty category
• partially ordered collection of ordinal numbers
• 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]
• walking commutative square
• walking composable pair
• walking span

Unknown

There is 1 category for which the database has no information on whether it satisfies this property. Please help us fill in the gaps by contributing to this project.

• category of Z-functors