Cartesian closed category
In category theory, a category is Cartesian closed if, roughly speaking, any morphism defined on a product of two objects can be naturally identified with a morphism defined on one of the factors. These categories are particularly important in mathematical logic and the theory of programming, in that their internal language is the simply typed lambda calculus. They are generalized by closed monoidal categories, whose internal language, linear type systems, are suitable for both quantum and classical computation.
Etymology
Named after , French philosopher, mathematician, and scientist, whose formulation of analytic geometry gave rise to the concept of Cartesian product, which was later generalized to the notion of categorical product.Definition
The category C is called Cartesian closed if and only if it satisfies the following three properties:- It has a terminal object.
- Any two objects X and Y of C have a product X ×Y in C.
- Any two objects Y and Z of C have an exponential ZY in C.
The third condition is equivalent to the requirement that the functor – ×Y has a right adjoint, usually denoted –Y, for all objects Y in C.
For locally small categories, this can be expressed by the existence of a bijection between the hom-sets
which is natural in both X and Z.
Take care to note that a Cartesian closed category need not have finite limits; only finite products are guaranteed.
If a category has the property that all its slice categories are Cartesian closed, then it is called locally cartesian closed. Note that if C is locally Cartesian closed, it need not actually be Cartesian closed; that happens if and only if C has a terminal object.
Basic constructions
Evaluation
For each object Y, the counit of the exponential adjunction is a natural transformationcalled the evaluation map. More generally, we can construct the partial application map as the composite
In the particular case of the category Set, these reduce to the ordinary operations:
Composition
Evaluating the exponential in one argument at a morphism p : X → Y gives morphismscorresponding to the operation of composition with p. Alternate notations for the operation pZ include p* and p∘-. Alternate notations for the operation Zp include p* and -∘p.
Evaluation maps can be chained as
the corresponding arrow under the exponential adjunction
is called the composition map.
In the particular case of the category Set, this is the ordinary composition operation:
Sections
For a morphism p:X → Y, suppose the following pullback square exists, which defines the subobject of XY corresponding to maps whose composite with p is the identity:where the arrow on the right is pY and the arrow on the bottom corresponds to the identity on Y. Then ΓY is called the object of sections of p. It is often abbreviated as ΓY.
If ΓY exists for every morphism p with codomain Y, then it can be assembled into a functor ΓY : C/Y → C on the slice category, which is right adjoint to a variant of the product functor:
The exponential by Y can be expressed in terms of sections:
Examples
Examples of Cartesian closed categories include:- The category Set of all sets, with functions as morphisms, is Cartesian closed. The product X×Y is the Cartesian product of X and Y, and ZY is the set of all functions from Y to Z. The adjointness is expressed by the following fact: the function f : X×Y → Z is naturally identified with the curried function g : X → ZY defined by g = f for all x in X and y in Y.
- The category of finite sets, with functions as morphisms, is Cartesian closed for the same reason.
- If G is a group, then the category of all G-sets is Cartesian closed. If Y and Z are two G-sets, then ZY is the set of all functions from Y to Z with G action defined by = g. for all g in G, F:Y → Z and y in Y.
- The category of finite G-sets is also Cartesian closed.
- The category Cat of all small categories is Cartesian closed; the exponential CD is given by the functor category consisting of all functors from D to C, with natural transformations as morphisms.
- If C is a small category, then the functor category SetC consisting of all covariant functors from C into the category of sets, with natural transformations as morphisms, is Cartesian closed. If F and G are two functors from C to Set, then the exponential FG is the functor whose value on the object X of C is given by the set of all natural transformations from to F.
- * The earlier example of G-sets can be seen as a special case of functor categories: every group can be considered as a one-object category, and G-sets are nothing but functors from this category to Set
- * The category of all directed graphs is Cartesian closed; this is a functor category as explained under functor category.
- * In particular, the category of simplicial sets is Cartesian closed.
- Even more generally, every elementary topos is Cartesian closed.
- In algebraic topology, Cartesian closed categories are particularly easy to work with. Neither the category of topological spaces with continuous maps nor the category of smooth manifolds with smooth maps is Cartesian closed. Substitute categories have therefore been considered: the category of compactly generated Hausdorff spaces is Cartesian closed, as is the category of Frölicher spaces.
- In order theory, complete partial orders have a natural topology, the Scott topology, whose continuous maps do form a Cartesian closed category. Both currying and apply are continuous functions in the Scott topology, and currying, together with apply, provide the adjoint.
- A Heyting algebra is a Cartesian closed lattice. An important example arises from topological spaces. If X is a topological space, then the open sets in X form the objects of a category O for which there is a unique morphism from U to V if U is a subset of V and no morphism otherwise. This poset is a Cartesian closed category: the "product" of U and V is the intersection of U and V and the exponential UV is the interior of.
- A category with a zero object is Cartesian closed if and only if it is equivalent to a category with only one object and one identity morphism. Indeed, if 0 is an initial object and 1 is a final object and we have, then which has only one element.
- *In particular, any non-trivial category with a zero object, such as an abelian category, is not Cartesian closed. So the category of modules over a ring is not Cartesian closed. However, the functor tensor product with a fixed module does have a right adjoint. The tensor product is not a categorical product, so this does not contradict the above. We obtain instead that the category of modules is monoidal closed.
- Every elementary topos is locally Cartesian closed. This example includes Set, FinSet, G-sets for a group G, as well as SetC for small categories C.
- The category LH whose objects are topological spaces and whose morphisms are local homeomorphisms is locally Cartesian closed, since LH/X is equivalent to the category of sheaves. However, LH does not have a terminal object, and thus is not Cartesian closed.
- If C has pullbacks and for every arrow p : X → Y, the functor p* : C/Y → C/X given by taking pullbacks has a right adjoint, then C is locally Cartesian closed.
- If C is locally Cartesian closed, then all of its slice categories C/X are also locally Cartesian closed.
- Cat is not locally Cartesian closed.
Applications
The Curry–Howard–Lambek correspondence provides a deep isomorphism between intuitionistic logic, simply-typed lambda calculus and Cartesian closed categories.
Certain Cartesian closed categories, the topoi, have been proposed as a general setting for mathematics, instead of traditional set theory.
The renowned computer scientist John Backus has advocated a variable-free notation, or Function-level programming, which in retrospect bears some similarity to the internal language of Cartesian closed categories. CAML is more consciously modelled on Cartesian closed categories.
Dependent sum and product
Let C be a locally Cartesian closed category. Then C has all pullbacks, because the pullback of two arrows with codomain Z is given by the product in C/Z.For every arrow p : X → Y, let P denote the corresponding object of C/Y. Taking pullbacks along p gives a functor p* : C/Y → C/X which has both a left and a right adjoint.
The left adjoint is called the dependent sum and is given by composition with p.
The right adjoint is called the dependent product.
The exponential by P in C/Y can be expressed in terms of the dependent product by the formula.
The reason for these names is because, when interpreting P as a dependent type, the functors and correspond to the type formations and respectively.
Equational theory
In every Cartesian closed category, Z and Y are isomorphic for all objects X, Y and Z. We write this as the "equation"One may ask what other such equations are valid in all Cartesian closed categories. It turns out that all of them follow logically from the following axioms:
- x× = ×z
- x×y = y×x
- x×1 = x
- 1x = 1
- x1 = x
- z = xz×yz
- z = x
Bicartesian closed categories
- x + y = y + x
- + z = x +
- x× = x×y + x×z
- x = xy×xz
- 0 + x = x
- x×0 = 0
- x0 = 1