Definition. Locally Discrete Bicategory

2026-09-29 ยท category-theory bicategory

Every category ๐’ž๏ธ€ is a bicategory ๐–ซ๐–ฃ(๐’ž๏ธ€) with only identity 2-cells. Its 0-cells are the objects of ๐’ž๏ธ€, and the hom-category ๐–ซ๐–ฃ(๐’ž๏ธ€)(๐‘ฅ,๐‘ฆ) is the discrete category on the set ๐’ž๏ธ€(๐‘ฅ,๐‘ฆ): a 2-cell ๐‘“โ‡’๐‘” is a proof that ๐‘“=๐‘”. Composition and identities are those of ๐’ž๏ธ€; the unitors and associator are the unit and associativity laws of ๐’ž๏ธ€. Since the homs of ๐’ž๏ธ€ are sets, any two parallel 2-cells are equal, so the triangle and pentagon hold trivially.

A functor ๐น:๐’ž๏ธ€โ†’๐’Ÿ๏ธ€ gives a pseudofunctor ๐–ซ๐–ฃ(๐น):๐–ซ๐–ฃ(๐’ž๏ธ€)โ†’๐–ซ๐–ฃ(๐’Ÿ๏ธ€), whose comparison 2-cells are the functor laws of ๐น.

The locally discrete bicategory is how ordinary indexed categories enter bicategorical language: a prestack on ๐–ซ๐–ฃ(๐’ž๏ธ€) is a pseudofunctor ๐’ž๏ธ€opโ†’๐–ข๐– ๐–ณ, and its Grothendieck construction is a displayed category over ๐’ž๏ธ€.

locally-discrete-bicategory definition entries/bicategory/locally-discrete-bicategory.hel