nLab weak adjoint

Weak adjoints

Weak adjoints


A weak adjoint is like an adjoint functor but without the uniqueness of factorizations.


Let G:DCG:D\to C be a functor. We say GG has a left weak adjoint if for each xCx\in C there exists a morphism η x:xGz\eta_x:x\to G z such that for any morphism f:xGyf:x\to G y there exists a (not necessarily unique) morphism g:yzg:y\to z such that f=Ggη xf = G g \circ\eta_x.


  • A weak limit is a weak right adjoint to a constant-diagram functor.


Of course, if the factorizations gg are always unique, this is precisely an ordinary left adjoint. A weak adjoint can also be regarded as a stronger form of the solution set condition in which the solution sets are required to be singletons. The “dual” property, in which the solution sets need not be singletons but the factorizations are unique, is called a multi-adjoint.

Created on March 7, 2021 at 16:19:44. See the history of this page for a list of all contributions to it.