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Thursday, March 7, 2013

Relations between the 2-categories Set-Cat and 2-Cat

There is a strict 2-functor (()Cat=()):MONCATCAT:V(VCat=V)
between (the indicated strict 2-categories); see CC II.6.3, also RE2C ¶1.3.2;
the notation VCat is now more standard, see e.g. BCECT,
but CC uses the briefer notations V, Φ and η for the effect on monoidal categories, monoidal functors and monoidal natural transformations.
(Also, CC calls 2-categories “hypercategories”, a term no longer used for 2-categories.)
(The () notation) is consistent with (other usages of ()):
if we severely overload (the symbol X) so that it stands for (a) a V-category and (b) that V-category's set (or class) of objects, with (c) X denoting X's hom-function,
then (the hom-function of XΦ) is defined as: (XΦ):X×XXVϕV:x,yxXyϕ=x(XΦ)y. For its brevity we adopt this notation for the effects of (()Cat=()) on monoidal functors and monoidal natural transformations: thus F and η,
while using the notation VCat for the effect on monoidal categories.

Recall (the monoidal adjunction in CAT), i.e., (the ordinary adjunction in the 2-category MONCAT): (F=f,˜f,f)(U=u,˜u,u):2,,Set,×,1 hXSet(2A)Set,×,1Set,×,1Set,×,1 2-functors take adjunctions to adjunctions (¶2.3.1 of RE2C), thus
[the image under the 2-functor \big( ()\mathord-\Cat = ()_\ast \big)]
of (the above adjunction in the 2-category \MONCAT)
is (an adjunction in the 2-category \CAT): \boxed{\begin{array}{} \functionh & \in & \mkern1em \hom \setX {\Set\mathord-\Cat} {\objectA \functorU_\ast} & \mkern2em \smash{\Rule {2px} {2ex} {11ex}} \mkern1em & && \langle \Set,\times,1 \rangle\mathord-\Cat && \xrightarrow{\mkern3em} && \langle \Set,\times,1 \rangle\mathord-\Cat && \xrightarrow{\mkern3em} && \langle \Set,\times,1 \rangle\mathord-\Cat \\ && \wr\Vert && & \llap\setX \nearrow & \llap\functionk \Big\Downarrow & \searrow \rlap{\functorF_\ast} & \Big\Downarrow \rlap{\eta_\ast} & \nearrow \rlap{\functorU_\ast} & \llap\Vert \Big\Downarrow \rlap{\epsilon_\ast} & \searrow \rlap{\functorF_\ast} & \Big\Downarrow \rlap{\eta_\ast} & \nearrow \rlap{\functorU_\ast = \langle \functoru,\tilde\functoru,\functoru^\circ \rangle _\ast} \\ \functionk & \in & \mkern.6em \hom {\setX \functorF_\ast} {\cattwo\mathord-\Cat} \objectA && \catI && \xrightarrow[\textstyle \objectA]{\smash{\mkern3em}} && \langle \cattwo,\land,\top \rangle\mathord-\Cat && \xrightarrow[\mkern3em]{\smash{}} && \langle \cattwo,\land,\top \rangle\mathord-\Cat \\ \end{array}} (The monoidal structures \functoru^\circ, \tilde\functoru, \functorf^\circ, and \tilde\functorf) combine with (the functors \functoru and \functorf themselves)
to produce (\functorU_\ast and \functorF_\ast):
the units (CC I(6.3)) and compositions (CC II(6.1)) of (\objectA\functorU_\ast and \objectX\functorF_\ast) are defined by: \mkern-3em \begin{array}{} && && && && & \hom \objecta {(\objectA\functorU_\ast)} \objectb \times \hom \objectb {(\objectA\functorU_\ast)} \objectc &&&& &&&& &&&& & \hom \objectx {(\objectX\functorF_\ast)} \objecty \land \hom \objecty {(\objectX\functorF_\ast)} \objectz \\ && && && && & \Vert &&&& &&&& &&&& & \Vert \\ && 1 & = & 1 && && & (\hom \objecta \objectA \objectb)\functoru \times (\hom \objectb \objectA \objectc)\functoru &&&&&& \top & = & \top &&&&& (\hom \objectx \objectX \objecty)\functorf \land (\hom \objecty \objectX \objectz)\functorf \\ && \llap{\wr\Vert} \big\downarrow & \functoru^\circ & \llap{\wr\Vert} \big\downarrow & && && \llap{\wr\Vert} \big\downarrow \rlap{\langle \hom \objecta \objectA \objectb, \hom \objectb \objectA \objectc \rangle \tilde\functoru} &&&& && \llap{\Vert} \big\downarrow & \functorf^\circ & \llap{\Vert} \big\downarrow &&&&& \llap{\Vert} \big\downarrow \rlap{\langle \hom \objectx \objectX \objecty, \hom \objecty \objectX \objectz \rangle \tilde\functorf} \\ \top & \mathop\mapsto\limits^\functoru& \top\functoru & = & \hom \top \cattwo \top &&& \hom \objecta \objectA \objectb \land \hom \objectb \objectA \objectc & \mathop\mapsto\limits^\functoru & \big( \hom \objecta \objectA \objectb \land \hom \objectb \objectA \objectc \big)\functoru &&&& 1 & \mathop\mapsto\limits^\functorf & 1\functorf & = & 1\cdot\top &&& \hom \objectx \objectX \objecty \times \hom \objecty \objectX \objectz & \mathop\mapsto\limits^\functorf & \big( \hom \objectx \objectX \objecty \times \hom \objecty \objectX \objectz \big)\functorf \\ \llap{\text{refl}} \big\downarrow & \mathop\mapsto\limits^\functoru & \llap{\text{refl}\functoru} \big\downarrow & = & \big\downarrow \rlap{\hom \top \cattwo {\text{refl}}} &&& \llap{\text{trans}} \big\downarrow & \mathop\mapsto\limits^\functoru & \llap{\text{trans}\functoru} \big\downarrow &&&& \llap{\objectx\arrowj} \big\downarrow & \mathop\mapsto\limits^\functorf & \llap{\objectx\arrowj\functorf} \big\downarrow & = & \big\downarrow \rlap{(\objectx\arrowj)\cdot\top} &&& \llap{\Mforcomp} \big\downarrow & \mathop\mapsto\limits^\functorf & \big\downarrow \rlap {\Mforcomp\functorf} \\ \hom \objecta \objectA \objecta & \mathop\mapsto\limits^\functoru & \hom \objecta \objectA \objecta \functoru & = & \hom \top \cattwo {\hom \objecta \objectA \objecta} &&& \hom \objecta \objectA \objectc & \mathop\mapsto\limits^\functoru & \hom \objecta \objectA \objectc \functoru &&&& \hom \objectx \objectX \objectx & \mathop\mapsto\limits^\functorf & \hom \objectx \objectX \objectx \functorf & = & \hom \objectx \objectX \objectx \cdot\top &&& \hom \objectx \objectX \objectz & \mathop\mapsto\limits^\functorf & \hom \objectx \objectX \objectz \functorf \\ && \Vert && && && & \Vert &&&& && \Vert && &&&& & \Vert \\ && {} \rlap{\mkern-2em \hom \objecta {(\objectA\functorU_\ast)} \objecta} && && && & {} \rlap{\mkern-2em \hom \objecta {(\objectA\functorU_\ast)} \objectc} &&&& && \hom \objectx {(\objectX\functorF_\ast)} \objectx && &&&& & \hom \objectx {(\objectX\functorF_\ast)} \objectz \\ \end{array}

Here of course (\text{refl} and \text{trans}) are the implementations in (\cattwo-category theory)
of (the reflexive and transitive axioms for preorders);
in (the notation of CC) they would be denoted \objecta\arrowj and (\Mforcomp = \Mforcomp^\objectb_{\objecta\objectc}) respectively.


References

CC, Eilenberg and Kelly, “Closed Categories”, 1966
RE2C, Kelly and Street, “Review of the Elements of 2-Categories”, 1974
BCECT, Kelly, Basic Concepts of Enriched Category Theory, 1982/2005

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