diff --git a/databases/catdat/data/functors/trivial_groups.yaml b/databases/catdat/data/functors/trivial_groups.yaml new file mode 100644 index 00000000..302d2b0e --- /dev/null +++ b/databases/catdat/data/functors/trivial_groups.yaml @@ -0,0 +1,27 @@ +id: trivial_groups +name: trivial functor from the category of groups +notation: $!$ +source: Grp +target: '1' +description: 'Every category $\C$ has a unique functor $! : \C \to 1$ into the trivial category. Here, we specify that $\C$ is the category of groups. It is a basic example of a full functor which is not faithful.' +nlab_link: null + +tags: + - algebra + +related_functors: + - trivial_sets + +satisfied_properties: + - property: coreflector + proof: 'The constant functor $1 \to \Grp$ with value the trivial group is a left adjoint of $! : \Grp \to 1$ and fully faithful; this is because the trivial group is an initial object of $\Grp$.' + + - property: reflector + proof: 'The constant functor $1 \to \Grp$ with value the trivial group is a left adjoint of $! : \Grp \to 1$ and fully faithful; this is because $\{\ast\}$ is a terminal object of $\Grp$.' + + - property: full + proof: This follows easily from the fact that $\Grp$ is strongly connected. + +unsatisfied_properties: + - property: essentially injective + proof: This is trivial. diff --git a/databases/catdat/data/functors/trivial_sets.yaml b/databases/catdat/data/functors/trivial_sets.yaml new file mode 100644 index 00000000..31d03551 --- /dev/null +++ b/databases/catdat/data/functors/trivial_sets.yaml @@ -0,0 +1,30 @@ +id: trivial_sets +name: trivial functor from the category of sets +notation: $!$ +source: Set +target: '1' +description: 'Every category $\C$ has a unique functor $! : \C \to 1$ into the trivial category. Here, we specify that $\C$ is the category of sets.' +nlab_link: null + +tags: + - set theory + +related_functors: + - trivial_groups + +satisfied_properties: + - property: coreflector + proof: 'The constant functor $1 \to \Set$ with value $\varnothing$ is a left adjoint of $! : \Set \to 1$ and fully faithful; this is because $\varnothing$ is an initial object of $\Set$.' + + - property: reflector + proof: 'The constant functor $1 \to \Set$ with value $\{\ast\}$ is a left adjoint of $! : \Set \to 1$ and fully faithful; this is because $\{\ast\}$ is a terminal object of $\Set$.' + +unsatisfied_properties: + - property: faithful + proof: 'Take any pair of distinct maps of sets $f,g : X \rightrightarrows Y$. They have the same image under the functor.' + + - property: full + proof: If $X$ is a non-empty set, there is a (unique) morphism ${!}(X) \to {!}(\varnothing)$, but no morphism $X \to \varnothing$. + + - property: essentially injective + proof: This is trivial.