\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏、\(F,G:\mathord {\textnormal {\textsf {Fun}}}(C,D)\)を関手とする。型\(\mathord {\textnormal {\textsf {NatTrans}}}(F,G):\mathcal {U}(i)\)を
[0065] 自然変換
自然変換は関手の間の射の概念である。実際、関手を対象、自然変換を射とする前圏を構成できる([0069])。さらに、終域が圏であるような関手のなす前圏は圏であることを示す([006C])。
\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏とする。
- 1関手\(F:\mathord {\textnormal {\textsf {Fun}}}(C,D)\)に対して、恒等自然変換\(\mathord {\textnormal {\textsf {id}}}\lbrace F\rbrace :\mathord {\textnormal {\textsf {NatTrans}}}(F,F)\)を\(\lambda x.\mathord {\textnormal {\textsf {id}}}\lbrace F(x)\rbrace \)と定義する。\(\mathord {\textnormal {\textsf {id}}}\lbrace F\rbrace \)の自然性を確認せよ。
- 2関手\(F_{1},F_{2},F_{3}:\mathord {\textnormal {\textsf {Fun}}}(C,D)\)と自然変換\(t_{1}:\mathord {\textnormal {\textsf {NatTrans}}}(F_{1},F_{2})\)と\(t_{2}:\mathord {\textnormal {\textsf {NatTrans}}}(F_{2},F_{3})\)に対して、合成\(t_{2}\circ t_{1}:\mathord {\textnormal {\textsf {NatTrans}}}(F_{1},F_{3})\)を\(\lambda x.t_{2}(x)\circ t_{1}(x)\)と定義する。\(t_{2}\circ t_{1}\)の自然性を確認せよ。
関数外延性を仮定する。\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏、\(F,G:\mathord {\textnormal {\textsf {Fun}}}(C,D)\)を関手、\(s,t:\mathord {\textnormal {\textsf {NatTrans}}}(F,G)\)を自然変換とすると、同値
関数外延性を仮定する。\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏とする。前圏\(\mathord {\textnormal {\textsf {Fun}}}^{(\mathord {\textnormal {\textsf {Cat}}})}(C,D):\mathord {\textnormal {\textsf {PreCat}}}(i)\)を次のように定義する。
関数外延性を仮定する。\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏、\(F,G:\mathord {\textnormal {\textsf {Fun}}}(C,D)\)を関手、\(t:\mathord {\textnormal {\textsf {NatTrans}}}(F,G)\)を自然変換とする。次は論理的に同値である。
証明
1から2を示す。各\(x:C\)に対して、射\(s(x),u(x):\mathord {\textnormal {\textsf {Map}}}(G(x),F(x))\)と同一視\(p(x):s(x)\circ t(x)=\mathord {\textnormal {\textsf {id}}}\)と\(q(x):t(x)\circ u(x)=\mathord {\textnormal {\textsf {id}}}\)を得る。[0068]より、\(s\)と\(u\)の自然性を確認すれば十分である。\(x_{1},x_{2}:C\)を対象、\(f:\mathord {\textnormal {\textsf {Map}}}(x_{1},x_{2})\)を射とする。\(F(f)\circ s(x_{1})=s(x_{2})\circ G(f)\)を言うには、[005G]より\(F(f)\circ s(x_{1})\circ t(x_{1})=s(x_{2})\circ G(f)\circ t(x_{1})\)を示せば十分で、これは次のように分かる。
関数外延性を仮定する。\(i\)を階数、\(C,D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏とする。\(D\)が圏ならば、\(\mathord {\textnormal {\textsf {Fun}}}^{(\mathord {\textnormal {\textsf {Cat}}})}(C,D)\)は圏である。
証明
\(F:\mathord {\textnormal {\textsf {Fun}}}^{(\mathord {\textnormal {\textsf {Cat}}})}(C,D)\)を対象とする。[006A]よりレトラクト\((\sum _{G:\mathord {\textnormal {\textsf {Fun}}}^{(\mathord {\textnormal {\textsf {Cat}}})}(C,D)}F\cong G)\triangleleft (\sum _{G:\mathord {\textnormal {\textsf {Fun}}}(C,D)}\lbrace t:\mathord {\textnormal {\textsf {NatTrans}}}(F,G)\mid \forall _{x:C}\mathord {\textnormal {\textsf {IsIso}}}(t(x))\rbrace )\)を得る。後者が可縮であることを示すには、[006B]より
\(\mathord {\textnormal {\textsf {Fun}}}^{(\mathord {\textnormal {\textsf {Cat}}})}(C,D)\)は普遍性でも特徴づけられる([006R])。
\(i\)を階数、\(C_{1},C_{2},D:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏とする。型\(\mathord {\textnormal {\textsf {BiFun}}}(C_{1},C_{2};D):\mathcal {U}(i)\)を次のレコード型と定義する。
- -\(\mathord {\textnormal {\textsf {obj}}}:C_{1}.\mathord {\textnormal {\textsf {Obj}}}\to C_{2}.\mathord {\textnormal {\textsf {Obj}}}\to D.\mathord {\textnormal {\textsf {Obj}}}\)
- -\(\mathord {\textnormal {\textsf {map}}}_{1}:\prod _{\lbrace x_{11},x_{12}:C_{1}\rbrace }\mathord {\textnormal {\textsf {Map}}}(x_{11},x_{12})\to (\prod _{x_{2}:C_{2}}\mathord {\textnormal {\textsf {Map}}}(\mathord {\textnormal {\textsf {obj}}}(x_{11},x_{2}),\mathord {\textnormal {\textsf {obj}}}(x_{12},x_{2})))\)
- -\(\mathord {\textnormal {\textsf {map}}}_{2}:\prod _{x_{1}:C_{1}}\prod _{\lbrace x_{21},x_{22}:C_{2}\rbrace }\mathord {\textnormal {\textsf {Map}}}(x_{21},x_{22})\to \mathord {\textnormal {\textsf {Map}}}(\mathord {\textnormal {\textsf {obj}}}(x_{1},x_{21}),\mathord {\textnormal {\textsf {obj}}}(x_{1},x_{22}))\)
- -\(\_:\forall _{x_{1}:C_{1}}\forall _{x_{2}:C_{2}}\mathord {\textnormal {\textsf {map}}}_{1}(\mathord {\textnormal {\textsf {id}}}\lbrace x_{1}\rbrace ,x_{2})=\mathord {\textnormal {\textsf {id}}}\lbrace \mathord {\textnormal {\textsf {obj}}}(x_{1},x_{2})\rbrace \)
- -\(\_:\forall _{x_{11},x_{12},x_{13}:C_{1}}\forall _{x_{2}:C_{2}}\forall _{f_{1}:\mathord {\textnormal {\textsf {Map}}}(x_{11},x_{12})}\forall _{f_{2}:\mathord {\textnormal {\textsf {Map}}}(x_{12},x_{13})}\mathord {\textnormal {\textsf {map}}}_{1}(f_{2}\circ f_{1},x_{2})=\mathord {\textnormal {\textsf {map}}}_{1}(f_{2},x_{2})\circ \mathord {\textnormal {\textsf {map}}}_{1}(f_{1},x_{2})\)
- -\(\_:\forall _{x_{1}:C_{1}}\forall _{x_{2}:C_{2}}\mathord {\textnormal {\textsf {map}}}_{2}(x_{1},\mathord {\textnormal {\textsf {id}}}\lbrace x_{2}\rbrace )=\mathord {\textnormal {\textsf {id}}}\lbrace \mathord {\textnormal {\textsf {obj}}}(x_{1},x_{2})\rbrace \)
- -\(\_:\forall _{x_{1}:C_{1}}\forall _{x_{21},x_{22},x_{23}:C_{2}}\forall _{f_{1}:\mathord {\textnormal {\textsf {Map}}}(x_{21},x_{22})}\forall _{f_{2}:\mathord {\textnormal {\textsf {Map}}}(x_{22},x_{23})}\mathord {\textnormal {\textsf {map}}}_{2}(x_{1},f_{2}\circ f_{1})=\mathord {\textnormal {\textsf {map}}}_{2}(x_{1},f_{2})\circ \mathord {\textnormal {\textsf {map}}}_{2}(x_{1},f_{1})\)
- -\(\_:\forall _{x_{11},x_{12}:C_{1}}\forall _{x_{21},x_{22}:C_{2}}\forall _{f_{1}:\mathord {\textnormal {\textsf {Map}}}(x_{11},x_{12})}\forall _{f_{2}:\mathord {\textnormal {\textsf {Map}}}(x_{21},x_{22})}\mathord {\textnormal {\textsf {map}}}_{1}(f_{1},x_{22})\circ \mathord {\textnormal {\textsf {map}}}_{2}(x_{11},f_{2})=\mathord {\textnormal {\textsf {map}}}_{2}(x_{12},f_{2})\circ \mathord {\textnormal {\textsf {map}}}_{1}(f_{1},x_{21})\)
関数外延性を仮定する。\(i\)を階数、\(C,D,X:\mathord {\textnormal {\textsf {PreCat}}}(i)\)を前圏とする。同値