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Constant enriched functors need not exist
Statement
There exist -categories whose underlying ordinary categories admit an ordinary constant functor, but no corresponding -functor with that constant object value. In particular, constant enriched functors do not exist in general.
Facts & Assumptions
Given: The base .
A -functor must preserve enriched identities and enriched composition (Enriched functor).
The underlying ordinary category keeps only global elements of the hom-object (The underlying ordinary category of an enriched category).
A -category is determined by its hom-objects together with identity and composition maps (Enriched category over a monoidal base).
Proof
Let be the one-object -category with hom-object , so its unique object has endomorphism object the tensor unit. Let be the one-object -category with hom-object , the terminal object of ; the identity map and the zero composition make this a valid -category by [L3].
The underlying ordinary category has one object and one morphism, because is a singleton by [L2]. The underlying ordinary category also has one object, with morphism set . Sending the unique object of to the unique object of and its identity to therefore defines an ordinary constant functor .
A -enriched functor would need a hom-object map preserving the enriched identity. But the identity in is the unique map , and the identity in is ; preserving identities would force the composite to be , impossible because the composite through is the zero homomorphism. This contradicts [L1].
Hence the ordinary constant functor of step 2.1 has no enriched lift, so constant enriched functors need not exist.
Depends on
Used by
- FALSE: every enriched category has constant enriched functors False statement
Dependency tree · two levels
4 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- G. M. Kelly, Basic Concepts of Enriched Category Theory, Section 3.9 (standard reference, not scraped)