The adjoint triangle theorem in category theory gives conditions under which, given a pair of functors and an adjoint functor, further adjoints exist.
Depending on the specific assumptions, the theorem has several variants. The following gives the most common formulation going back to Dubuc (1968).
Suppose that $U \colon B\to C$ is a functor which has a left adjoint $F \,\colon\, C\to B$ with the property that the diagram
is a pointwise coequalizer (i.e. $U$ is of descent type). Then for $A$ a category with coequalizers of reflexive pairs, a functor $R \colon A\to B$ has a left adjoint if and only if the composite $U R$ does.
The direction “only if” is obvious since adjunctions compose. For “if”, let $F'$ be a left adjoint of $U R$, and define $L:B\to A$ to be the pointwise coequalizer of
and
where $\theta:F \to R F'$ is the mate of the equality $U R = U R$ under the adjunctions $F\dashv U$ and $F'\dashv U R$. One then verifies that this works.
The hypotheses on $U$ are satisfied whenever it is monadic.
In fact, it suffices to assume that each counit $\epsilon : F U b \to b$ is a regular epimorphism, rather than it is the coequalizer of a specific given pair of maps. See (Street-Verity), Lemma 2.1.
Similarly, the adjoint lifting theorem states conditions on a square of functors in order to ensure the existence of certain adjoints. Since a triangle can be viewed as a square with ‘two sides composed’, it is possible to deduce the adjoint lifting theorem from the adjoint triangle theorem as a corollary.
It is also possible to derive the monadicity theorem from the adjoint triangle theorem Dubuc (1968).
Michael Barr, Charles Wells, section 3.7, pp.131 in: Toposes, Triples, and Theories, Springer (1985), Reprints in Theories and Applications of Categories 12 (2005) 1-287 [tac:tr12]
Eduardo Dubuc, Adjoint triangles, pp.69-81 in LNM 61 Springer Heidelberg 1968. [doi:10.1007/BFb0077118]
I. B. Im, G. M. Kelly, Adjoint-Triangle Theorems for Conservative Functors, Bulletin of the Australian Mathematical Society 36 1 (1987) pp.133-136. [doi:10.1017/S000497270002637X]
John Power, A unified approach to the lifting of adjoints, Cahiers de Topologie et Géométrie Différentielle Catégoriques 29 1 (1988) 67-77. (numdam)
Ross Street, Dominic Verity, The comprehensive factorization and torsors, Theory and Applications of Categories 23 3 (2010) 42-75. (TAC)
Walter Tholen, Adjungierte Dreiecke, Colimites und Kan-Erweiterungen, Mathematische Annalen 217 (1975) pp.121-129. (gdz)
Generalizations of the adjoint triangle theorem to 2-categories are considered in
Fernando Lucatelli Nunes, On biadjoint triangles, Theory and Applications of Categories 31 9 (2016) 217-256. TAC
Fernando Lucatelli Nunes, On lifting of biadjoints and lax algebras, General Algebraic Structures with Applications 9 1 (2018) 29-58. [doi:10.29252/CGASA.9.1.29, arXiv:1607.03087]
Last revised on November 16, 2023 at 08:42:08. See the history of this page for a list of all contributions to it.