Some free constructions in realizability and proof theory Original Research Article

Some old and new constructions of free categories with good properties (regularity, exactness, etc.) are investigated, consistently showing their role in proof theory and in realizability theory, and in particular in the construction of the “effective topos” of M. Hyland. The subject of “small complete categories” is discussed, with a proposed new definition of what “complete” should mean for a full reflective subcategory of a topos.