This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. Please help to improve this article by introducing more precise citations. (October 2009) (Learn how and when to remove this template message)

In mathematics, especially in order theory, a **complete Heyting algebra** is a Heyting algebra that is complete as a lattice. Complete Heyting algebras are the objects of three different categories; the category **CHey**, the category **Loc** of **locales**, and its opposite, the category **Frm** of frames. Although these three categories contain the same objects, they differ in their morphisms, and thus get distinct names. Only the morphisms of **CHey** are homomorphisms of complete Heyting algebras.

Locales and frames form the foundation of pointless topology, which, instead of building on point-set topology, recasts the ideas of general topology in categorical terms, as statements on frames and locales.

Consider a partially ordered set (*P*, ≤) that is a complete lattice. Then *P* is a **complete Heyting algebra** or **frame** if any of the following equivalent conditions hold:

*P*is a Heyting algebra, i.e. the operation has a right adjoint (also called the lower adjoint of a (monotone) Galois connection), for each element*x*of*P*.

- For all elements
*x*of*P*and all subsets*S*of*P*, the following infinite distributivity law holds:

*P*is a distributive lattice, i.e., for all*x*,*y*and*z*in*P*, we have

- and the meet operations are Scott continuous (i.e., preserve the suprema of directed sets) for all
*x*in*P*.

The entailed definition of Heyting implication is

Using a bit more category theory, we can equivalently define a frame to be a cocomplete cartesian closed poset.

The system of all open sets of a given topological space ordered by inclusion is a complete Heyting algebra.

The objects of the category **CHey**, the category **Frm** of frames and the category **Loc** of locales are complete Heyting algebras. These categories differ in what constitutes a morphism:

- The morphisms of
**Frm**are (necessarily monotone) functions that preserve finite meets and arbitrary joins.

- The definition of Heyting algebras crucially involves the existence of right adjoints to the binary meet operation, which together define an additional implication operation. Thus, the morphisms of
**CHey**are morphisms of frames that in addition preserves implication.

- The morphisms of
**Loc**are opposite to those of**Frm**, and they are usually called maps (of locales).

The relation of locales and their maps to topological spaces and continuous functions may be seen as follows. Let be any map. The power sets *P*(*X*) and *P*(*Y*) are complete Boolean algebras, and the map is a homomorphism of complete Boolean algebras. Suppose the spaces *X* and *Y* are topological spaces, endowed with the topology *O*(*X*) and *O*(*Y*) of open sets on *X* and *Y*. Note that *O*(*X*) and *O*(*Y*) are subframes of *P*(*X*) and *P*(*Y*). If is a continuous function, then
preserves finite meets and arbitrary joins of these subframes. This shows that *O* is a functor from the category **Top** of topological spaces to **Loc**, taking any continuous map

to the map

in **Loc** that is defined in **Frm** to be the inverse image frame homomorphism

Given a map of locales in **Loc**, it is common to write for the frame homomorphism that defines it in **Frm**. Using this notation, is defined by the equation

Conversely, any locale *A* has a topological space *S*(*A*), called its *spectrum*, that best approximates the locale. In addition, any map of locales determines a continuous map Moreover this assignment is functorial: letting *P*(1) denote the locale that is obtained as the power set of the terminal set the points of *S*(*A*) are the maps in **Loc**, i.e., the frame homomorphisms

For each we define as the set of points such that It is easy to verify that this defines a frame homomorphism whose image is therefore a topology on *S*(*A*). Then, if is a map of locales, to each point we assign the point defined by letting be the composition of with hence obtaining a continuous map This defines a functor from **Loc** to **Top**, which is right adjoint to *O*.

Any locale that is isomorphic to the topology of its spectrum is called *spatial*, and any topological space that is homeomorphic to the spectrum of its locale of open sets is called *sober*. The adjunction between topological spaces and locales restricts to an equivalence of categories between sober spaces and spatial locales.

Any function that preserves all joins (and hence any frame homomorphism) has a right adjoint, and, conversely, any function that preserves all meets has a left adjoint. Hence, the category **Loc** is isomorphic to the category whose objects are the frames and whose morphisms are the meet preserving functions whose left adjoints preserve finite meets. This is often regarded as a representation of **Loc**, but it should not be confused with **Loc** itself, whose morphisms are formally the same as frame homomorphisms in the opposite direction.

- P. T. Johnstone,
*Stone Spaces*, Cambridge Studies in Advanced Mathematics 3, Cambridge University Press, Cambridge, 1982. (ISBN 0-521-23893-5)

*Still a great resource on locales and complete Heyting algebras.*

- G. Gierz, K. H. Hofmann, K. Keimel, J. D. Lawson, M. Mislove, and D. S. Scott,
*Continuous Lattices and Domains*, In*Encyclopedia of Mathematics and its Applications*, Vol. 93, Cambridge University Press, 2003. ISBN 0-521-80338-1

*Includes the characterization in terms of meet continuity.*

- Francis Borceux:
*Handbook of Categorical Algebra III*, volume 52 of*Encyclopedia of Mathematics and its Applications*. Cambridge University Press, 1994.

*Surprisingly extensive resource on locales and Heyting algebras. Takes a more categorical viewpoint.*

- Steven Vickers,
*Topology via logic*, Cambridge University Press, 1989, ISBN 0-521-36062-5.

- Pedicchio, Maria Cristina; Tholen, Walter, eds. (2004).
*Categorical foundations. Special topics in order, topology, algebra, and sheaf theory*. Encyclopedia of Mathematics and Its Applications. Vol. 97. Cambridge: Cambridge University Press. ISBN 0-521-83414-7. Zbl 1034.18001.