Past exam of the mathematics course of the University of Cambridge / 2018 / iii / Paper 119 / 4 / ii

For $L⊣R$ inducing $T$, the Kleisli comparison functor is always full and faithful, since the adjunction givesTherefore it is part of an equivalence of categories if and only if every object of $\mathcal{D}$ is isomorphic to $LA$ for some $A$.

For an idempotent monad, multiplication is invertible. The unit laws imply $T\eta =\eta T={\mu }^{-1}$. If $(A,a)$ is an algebra for a monad, then $a{\eta }_A=1_A$, while naturality of $\eta$ givesThus $a:TA\to A$ is an isomorphism with inverse ${\eta }_A$. The algebra law $a{\mu }_A=aT(a)$ says that $a$ is a morphism of algebras for a monad from the free algebra $(TA,{\mu }_A)$ to $(A,a)$, so every object of the Eilenberg-Moore category is isomorphic to a free algebra. The criterion above yields