Abstract:
For functors $L:\mathbb{A}\to \mathbb{B}$ and $R:\mathbb{B}\to \mathbb{A}$ between any
categories $\mathbb{A}$ and $\mathbb{B}$,
a pairing is defined by maps, natural in $A\in \mathbb{A}$ and $B\in \mathbb{B}$,
$$
\xymatrix{{\rm Mor}_\mathbb{B} (L(A),B) \ar@<0.5ex>[r]^{\alpha} &
{\rm Mor}_\mathbb{A} (A,R(B))\ar@<0.5ex>[l]^{\beta}}.
$$
$(L,R)$ is an adjoint pair provided $\alpha$ (or $\beta$) is a bijection.
In this case the composition $RL$ defines a monad on the category $\mathbb{A}$, $LR$ defines a comonad
on the category $\mathbb{B}$, and there is a well-known correspondence between monads (or comonads)
and adjoint pairs of functors.
For various applications it was observed that the conditions for a unit
of a monad was too restrictive and weakening it still allowed for
a useful generalised notion of a monad. This led to the introduction of
weak monads and weak comonads and the definitions needed were made
without referring to this kind of adjunction.
The motivation for the present paper is to show that these notions can be naturally derived from
pairings of functors $(L,R,\alpha,\beta)$ with $\alpha = \alpha\cdot \beta\cdot \alpha$ and
$\beta = \beta \cdot\alpha\cdot\beta$.
Following closely the constructions known for monads (and unital modules) and
comonads (and counital comodules),
we show that any weak (co)monad on $\mathbb{A}$ gives rise
to a regular pairing between $\mathbb{A}$ and the category of compatible (co)modules.
Keywords:pairing of functors; adjoint functors; weak monads and comonads; $r$-unital monads; $r$-counital comonads; lifting of functors; distributive laws.