# nLab Gabriel-Ulmer duality

GabrielUlmer duality

category theory

# Gabriel–Ulmer duality

## The idea

Gabriel–Ulmer duality says that there is an equivalence of 2-categories (or in other words, a biequivalence)

$\begin{matrix} Lex^{op} & \to & LFP \\ C & \mapsto & Lex(C, Set) \end{matrix}$

where Lex is the 2-category of:

and LFP is the 2-category of

The idea is that an object $C \in Lex$ can be thought of as an essentially algebraic theory, which has a category of models $Lex(C,Set)$. Gabriel–Ulmer duality says that this category of models is locally finitely presentable, all LFP categories arise in this way, and we can recover the theory $C$ from its category of models. There are similar dualities for other classes of theory such as regular theories.

A version of Gabriel–Ulmer duality for enriched category theory was proved by Max Kelly (see LackTendas). Let $\mathcal{V}$ be a symmetric monoidal closed complete and cocomplete category which is locally finitely presentable as a closed category. Then let $\mathcal{V}$-$Lex$ be the 2-category of finitely complete $\mathcal{V}$-categories ($\mathcal{V}$-categories with finite weighted limits), finite limit preserving $\mathcal{V}$-functors, and $\mathcal{V}$-natural transformations, and $\mathcal{V}$-$LFP$ the 2-category of locally finitely presentable $\mathcal{V}$-categories, right adjoint $\mathcal{V}$-functors that preserve filtered colimits, and $\mathcal{V}$-natural transformations. Then there is a biequivalence

$\begin{matrix} \mathcal{V}-Lex^{op} & \to & \mathcal{V}-LFP \\ C & \mapsto & Lex(C, \mathcal{V}). \end{matrix}$

For instance, in the truth value-enriched case, the duality is between meet semilattices and algebraic lattices.

Gabriel-Ulmer duality is a duality exhibited by the 2-Chu construction, $Chu(Cat,Set)$.

The original source is:

A careful discussion and proof of the biequivalence is in

• Jiri Adamek, Hans-Eberhard Porst, Algebraic Theories of Quasivarieties , J. Algebra 208 (1998) pp.379-398.

Some other general treatments of Gabriel-Ulmer duality (and generalizations to other doctrines):

• C. Centazzo, E. M. Vitale, A duality relative to a limit doctrine, Theory and Appl. of Categories 10, No. 20, 2002, 486–497, pdf

• Stephen Lack, John Power, Gabriel–Ulmer duality and Lawvere Theories enriched over a general base, pdf

• M. Makkai, A. Pitts, Some results on locally finitely presentable categories, Trans. Amer. Math. Soc. 299 (1987), 473-496, MR88a:03162, doi, pdf

For a 2-dimensional analogue see the slides from a 2010 talk by Makkai: pdf

A formal-categorical account using Yoneda structures can be found in

For a discussion of Gabriel–Ulmer duality and related dualities in the context of enriched category theory see

This discusses (see Theorem 2.1) Kelly’s original result for $V$-enriched categories, where $V$ is a closed symmetric monoidal category whose underlying category $V_0$ is locally small, complete and cocomplete, in section 9 (cf. theorem 9.8) of

• Max Kelly, Structures defined by finite limits in the enriched context, Cahiers de Topologie et Géométrie Différentielle catégoriques, 23 no. 1 (1982), pp. 3-42, MR648793,numdam

For a connection to Tannaka duality theory see

For a discussion of an $\infty$-version of Gabriel-Ulmer duality between finitely complete and idempotent complete $(\infty, 1)$-categories and locally finitely presentable $(\infty, 1)$-categories see this MO discussion.