Just as universal arrows can be characterized as initial objects of some appropriate comma category, and (co)limits can be characterized as (initial) terminal objects of the appropriate (co)cone category, is there some (and if so, what is the) appropriate category for which, given a functor with a left/right adjoint, we can characterize said adjoint as its initial/terminal object? My goal in this is to characterize [initial objects, universals, limits, adjunctions] all as examples of each other.

  • $\begingroup$ it would be interesting to see all those connections in one place somewhere if you managed to write it down $\endgroup$ – nicolas Jan 16 '16 at 8:39

Well, yes: the left adjoint of a functor $G: C \to D$ is the initial object in the category whose objects are pairs $(H: D \to C, \eta: 1_D \to G H)$ where $\eta$ is a natural transformation, and whose morphisms $(H, \eta) \to (H', \eta')$ are natural transformations $\theta: H \to H'$ such that

$$\begin{array}{ccc} & 1_D & \\\\ {}^{ \eta} \swarrow & & \searrow {}^{\eta'} \\\\ GH & \xrightarrow{G\theta} & GH'\end{array}$$

commutes. Similarly, a right adjoint can be expressed as a terminal object in a suitable category (exercise in applying the concept of duality).

See the discussion on comma categories in Categories for the Working Mathematician,

  • $\begingroup$ In other (but essentally the some) words, considering the lax-comma 2-category $CAt // D$ the left adjoint of $G$ is the initial object of the Hom-category $CAt // D[1_D, G]$. $\endgroup$ – Buschi Sergio Nov 25 '12 at 8:45
  • $\begingroup$ Feel free to erase the commutative diagram if you don't like it. $\endgroup$ – Martin Brandenburg Nov 25 '12 at 10:17
  • $\begingroup$ or in tom leinster book basic category theory, that's his 3rd presentation of adjunction $\endgroup$ – nicolas Jan 16 '16 at 8:40

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