remark. idempotent [zhang2021type, 5.30] [tt-0037]

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Given an adjunction \(\mathscr {L} \dashv \mathscr {R}: {\cal C} \rightleftarrows {\cal D}\), we may obtain two endofunctors \( \mathscr {L} \mathbin {\bullet } \mathscr {R} : {\cal C} \to {\cal C}\) and \( \mathscr {R} \mathbin {\bullet } \mathscr {L} : {\cal D} \to {\cal D}\) that commute the diagram that means they are both idempotent, i.e. applying \(\mathscr {L} \mathbin {\bullet } \mathscr {R}\) any times yields the same result as applying it once, and similarly for \(\mathscr {R} \mathbin {\bullet } \mathscr {L}\).