Galois connection

In mathematics, especially in order theory, a Galois connection is a particular correspondence (typically) between two partially ordered sets (posets). The same notion can also be defined on preordered sets or classes; this article presents the common case of posets. Galois connections generalize the correspondence between subgroups and subfields investigated in Galois theory (named after the French mathematician Évariste Galois). They find applications in various mathematical theories.
A Galois connection is rather weak compared to an order isomorphism between the involved posets, but every Galois connection gives rise to an isomorphism of certain sub-posets, as will be explained below.
The literature contains two closely related notions of "Galois connection". In this article, we will distinguish between the two by referring to the first as (monotone) Galois connection and to the second as antitone Galois connection.
The term Galois correspondence is sometimes used to mean bijective "Galois connection"; this is simply an order isomorphism (or dual order isomorphism, depending on whether we take monotone or antitone Galois connections).
Definitions.
(Monotone) Galois connection.
Let and be two partially ordered sets. A "monotone Galois connection" between these posets consists of two monotone functions: and , such that for all in and in , we have
In this situation, is called the lower adjoint of and is called the upper adjoint of "F". Mnemonically, the upper/lower terminology refers to where the function application appears relative to ≤. The term "adjoint" refers to the fact that monotone Galois connections are special cases of pairs of adjoint functors in category theory as discussed further below. Other terminology encountered here is left adjoint (respectively right adjoint) for the lower (respectively upper) adjoint.
An essential property of a Galois connection is that an upper/lower adjoint of a Galois connection "uniquely" determines the other:
A consequence of this is that if or is invertible, then each is the inverse of the other, i.e. .
Given a Galois connection with lower adjoint and upper adjoint , we can consider the compositions , known as the associated closure operator, and , known as the associated kernel operator. Both are monotone and idempotent, and we have for all in and for all in .
A Galois insertion of into is a Galois connection in which the kernel operator is the identity on , and hence is an order-isomorphism of onto the closed sets [] of .
Antitone Galois connection.
The above definition is common in many applications today, and prominent in lattice and domain theory. However the original notion in Galois theory is slightly different. In this alternative definition, a Galois connection is a pair of "antitone", i.e. order-reversing, functions and between two posets and , such that
The symmetry of and in this version erases the distinction between upper and lower, and the two functions are then called polarities rather than adjoints. Each polarity uniquely determines the other, since
The compositions and are the associated closure operators; they are monotone idempotent maps with the property for all in and for all in .
The implications of the two definitions of Galois connections are very similar, since an antitone Galois connection between and is just a monotone Galois connection between and the order dual of . All of the below statements on Galois connections can thus easily be converted into statements about antitone Galois connections.
Examples.
Monotone Galois connections.
Power set; implication and conjunction.
For an order theoretic example, let be some set, and let and both be the power set of , ordered by inclusion. Pick a fixed subset of . Then the maps and , where , and , form a monotone Galois connection, with being the lower adjoint. A similar Galois connection whose lower adjoint is given by the meet (infimum) operation can be found in any Heyting algebra. Especially, it is present in any Boolean algebra, where the two mappings can be described by and . In logical terms: "implication from " is the upper adjoint of "conjunction with ".
Lattices.
Further interesting examples for Galois connections are described in the article on completeness properties. Roughly speaking, it turns out that the usual functions ∨ and ∧ are lower and upper adjoints to the diagonal map . The least and greatest elements of a partial order are given by lower and upper adjoints to the unique function Going further, even complete lattices can be characterized by the existence of suitable adjoints. These considerations give some impression of the ubiquity of Galois connections in order theory.
Transitive group actions.
Let act transitively on and pick some point in . Consider
the set of blocks containing . Further, let formula_2 consist of the subgroups of containing the stabilizer of .
Then, the correspondence formula_3:
is a monotone, one-to-one Galois connection. As a corollary, one can establish that doubly transitive actions have no blocks other than the trivial ones (singletons or the whole of ): this follows from the stabilizers being maximal in in that case. See doubly transitive group for further discussion.
Image and inverse image.
If is a function, then for any subset of we can form the image and for any subset of we can form the inverse image Then and form a monotone Galois connection between the power set of and the power set of , both ordered by inclusion ⊆. There is a further adjoint pair in this situation: for a subset of , define Then and form a monotone Galois connection between the power set of and the power set of . In the first Galois connection, is the upper adjoint, while in the second Galois connection it serves as the lower adjoint.
In the case of a quotient map between algebraic objects (such as groups), this connection is called the lattice theorem: subgroups of connect to subgroups of , and the closure operator on subgroups of is given by .
Span and closure.
Pick some mathematical object that has an underlying set, for instance a group, ring, vector space, etc. For any subset of , let be the smallest subobject of that contains , i.e. the subgroup, subring or subspace generated by . For any subobject of , let be the underlying set of . (We can even take to be a topological space, let the closure of , and take as "subobjects of " the closed subsets of .) Now and form a monotone Galois connection between subsets of and subobjects of , if both are ordered by inclusion. is the lower adjoint.
Syntax and semantics.
A very general comment of William Lawvere is that "syntax and semantics" are adjoint: take to be the set of all logical theories (axiomatizations), and the power set of the set of all mathematical structures. For a theory , let be the set of all structures that satisfy the axioms ; for a set of mathematical structures , let be the minimum of the axiomatizations which approximate . We can then say that is a subset of if and only if logically implies : the "semantics functor" and the "syntax functor" form a monotone Galois connection, with semantics being the lower adjoint.
Antitone Galois connections.
Galois theory.
The motivating example comes from Galois theory: suppose is a field extension. Let be the set of all subfields of that contain , ordered by inclusion ⊆. If is such a subfield, write for the group of field automorphisms of that hold fixed. Let be the set of subgroups of , ordered by inclusion ⊆. For such a subgroup , define to be the field consisting of all elements of that are held fixed by all elements of . Then the maps and form an antitone Galois connection.
Algebraic topology: covering spaces.
Analogously, given a path-connected topological space , there is an antitone Galois connection between subgroups of the fundamental group and path-connected covering spaces of . In particular, if is semi-locally simply connected, then for every subgroup of , there is a covering space with as its fundamental group.
Linear algebra: annihilators and orthogonal complements.
Given an inner product space , we can form the orthogonal complement of any subspace of . This yields an antitone Galois connection between the set of subspaces of and itself, ordered by inclusion; both polarities are equal to .
Given a vector space and a subset of we can define its annihilator , consisting of all elements of the dual space of that vanish on . Similarly, given a subset of , we define its annihilator This gives an antitone Galois connection between the subsets of and the subsets of .
Algebraic geometry.
In algebraic geometry, the relation between sets of polynomials and their zero sets is an antitone Galois connection.
Fix a natural number and a field and let be the set of all subsets of the polynomial ring ordered by inclusion ⊆, and let be the set of all subsets of ordered by inclusion ⊆. If is a set of polynomials, define the variety of zeros as
the set of common zeros of the polynomials in . If is a subset of , define as an ideal of polynomials vanishing on , that is
Then and "I" form an antitone Galois connection.
The closure on is the closure in the Zariski topology, and if the field is algebraically closed, then the closure on the polynomial ring is the radical of ideal generated by .
More generally, given a commutative ring (not necessarily a polynomial ring), there is an antitone Galois connection between radical ideals in the ring and subvarieties of the affine variety .
More generally, there is an antitone Galois connection between ideals in the ring and subschemes of the corresponding affine variety.
Connections on power sets arising from binary relations.
Suppose and are arbitrary sets and a binary relation over and is given. For any subset of , we define Similarly, for any subset of , define Then and yield an antitone Galois connection between the power sets of and , both ordered by inclusion ⊆.
Up to isomorphism "all" antitone Galois connections between power sets arise in this way. This follows from the "Basic Theorem on Concept Lattices". Theory and applications of Galois connections arising from binary relations are studied in formal concept analysis. That field uses Galois connections for mathematical data analysis. Many algorithms for Galois connections can be found in the respective literature, e.g., in.
Properties.
In the following, we consider a (monotone) Galois connection , where is the lower adjoint as introduced above. Some helpful and instructive basic properties can be obtained immediately. By the defining property of Galois connections, is equivalent to , for all in . By a similar reasoning (or just by applying the duality principle for order theory), one finds that , for all in . These properties can be described by saying the composite is "deflationary", while is "inflationary" (or "extensive").
Now consider such that , then using the above one obtains . Applying the basic property of Galois connections, one can now conclude that . But this just shows that preserves the order of any two elements, i.e. it is monotone. Again, a similar reasoning yields monotonicity of . Thus monotonicity does not have to be included in the definition explicitly. However, mentioning monotonicity helps to avoid confusion about the two alternative notions of Galois connections.
Another basic property of Galois connections is the fact that , for all in . Clearly we find that
because is inflationary as shown above. On the other hand, since is deflationary, while is monotonic, one finds that
This shows the desired equality. Furthermore, we can use this property to conclude that
and
i.e., and are idempotent.
It can be shown (see Blyth or Erné for proofs) that a function is a lower (resp. upper) adjoint if and only if is a residuated mapping (resp. residual mapping). Therefore, the notion of residuated mapping and monotone Galois connection are essentially the same.
Closure operators and Galois connections.
The above findings can be summarized as follows: for a Galois connection, the composite is monotone (being the composite of monotone functions), inflationary, and idempotent. This states that is in fact a closure operator on . Dually, is monotone, deflationary, and idempotent. Such mappings are sometimes called kernel operators. In the context of frames and locales, the composite is called the nucleus induced by . Nuclei induce frame homomorphisms; a subset of a locale is called a sublocale if it is given by a nucleus.
Conversely, any closure operator on some poset gives rise to the Galois connection with lower adjoint being just the corestriction of to the image of (i.e. as a surjective mapping the closure system ). The upper adjoint is then given by the inclusion of into , that maps each closed element to itself, considered as an element of . In this way, closure operators and Galois connections are seen to be closely related, each specifying an instance of the other. Similar conclusions hold true for kernel operators.
The above considerations also show that closed elements of (elements with ) are mapped to elements within the range of the kernel operator , and vice versa.
Existence and uniqueness of Galois connections.
Another important property of Galois connections is that lower adjoints preserve all suprema that exist within their domain. Dually, upper adjoints preserve all existing infima. From these properties, one can also conclude monotonicity of the adjoints immediately. The adjoint functor theorem for order theory states that the converse implication is also valid in certain cases: especially, any mapping between complete lattices that preserves all suprema is the lower adjoint of a Galois connection.
In this situation, an important feature of Galois connections is that one adjoint uniquely determines the other. Hence one can strengthen the above statement to guarantee that any supremum-preserving map between complete lattices is the lower adjoint of a unique Galois connection. The main property to derive this uniqueness is the following: For every in , is the least element of such that . Dually, for every in , is the greatest in such that . The existence of a certain Galois connection now implies the existence of the respective least or greatest elements, no matter whether the corresponding posets satisfy any completeness properties. Thus, when one upper adjoint of a Galois connection is given, the other upper adjoint can be defined via this same property.
On the other hand, some monotone function is a lower adjoint if and only if each set of the form for in , contains a greatest element. Again, this can be dualized for the upper adjoint.
Galois connections as morphisms.
Galois connections also provide an interesting class of mappings between posets which can be used to obtain categories of posets. Especially, it is possible to compose Galois connections: given Galois connections between posets and and between and , the composite is also a Galois connection. When considering categories of complete lattices, this can be simplified to considering just mappings preserving all suprema (or, alternatively, infima). Mapping complete lattices to their duals, these categories display auto duality, that are quite fundamental for obtaining other duality theorems. More special kinds of morphisms that induce adjoint mappings in the other direction are the morphisms usually considered for frames (or locales).
Connection to category theory.
Every partially ordered set can be viewed as a category in a natural way: there is a unique morphism from "x" to "y" if and only if . A monotone Galois connection is then nothing but a pair of adjoint functors between two categories that arise from partially ordered sets. In this context, the upper adjoint is the "right adjoint" while the lower adjoint is the "left adjoint". However, this terminology is avoided for Galois connections, since there was a time when posets were transformed into categories in a dual fashion, i.e. with arrows pointing in the opposite direction. This led to a complementary notation concerning left and right adjoints, which today is ambiguous.
Applications in the theory of programming.
Galois connections may be used to describe many forms of abstraction in the theory of abstract interpretation of programming languages.
References.
"The following books and survey articles include Galois connections using the monotone definition:"
"Some publications using the original (antitone) definition:"