Limit-creating functor 2026-10-07
A functor creates categorical limits of a specified shape when every given limiting categorical cone over an image diagram uniquely lifts to a categorical cone over the original diagram, and the lift is limiting. This is stronger than just being a limit-reflecting functor. Forgetting arrows from a functor category creates its pointwise limits in a functor category.
Past exam of the mathematics course of the University of Cambridge 2013 iii Paper 18 3 a Solution Created 2026-10-03 Updated 2026-10-07
For a diagram , a limit-preserving functor carries every limiting cone over a diagram to a limiting categorical cone. A limit-reflecting functor has the converse property: a categorical cone is limiting whenever its image is limiting. A limit-creating functor uniquely lifts every specified limiting categorical cone over the image diagram to a categorical cone over , and the lift is limiting. These definitions concern diagrams of the stipulated shape; creation includes the lifting requirement, not merely reflection.
Let be a categorical limit of . The mapis a bijection: the right side consists exactly of compatible families of arrows from , and the categorical limit's universal property gives their unique factorization through . The bijection is induced by the categorical limit projections, so it proves that covariant representables preserve limits, including the empty diagram and its terminal object.