We determine the existential completion of a primary doctrine, and we prove that the 2-monad obtained from it is lax-idempotent, and that the 2-category of existential doctrines is isomorphic to the 2-category of algebras for this 2-monad. We also show that the existential completion of an elementary doctrine is again elementary. Finally we extend the notion of exact completion of an elementary existential doctrine to an arbitrary elementary doctrine.
Keywords: existential completion, doctrines, property-like monad, tripos
2020 MSC: 18C10, 18C15, 18D05
Theory and Applications of Categories, Vol. 35, 2020, No. 43, pp 1576-1607.