The rudiments of the theoly of Galois connections (or residuation theory, as it is sometimes called) are provided, together with many examples and applications. Galois connections occur in profusion and are well known to most mathematicians who deal with order theory; they seem to be less known to topologists. However, because of their ubiquity and simplicity, they (like equivalence relations) can be used as an effective research tool throughout mathematics and related areas. If one recognizes that a Galois connection is involved in a phenomenon that may be relatively complex, then many aspects of that phenomenon immediately become clear, and thus, the whole situation typically becomes much easier to understand.
Mathematics SubjectClassification. Primary 06A15, 06-01, 06A06; Secondary 54-01, 54B99, 54899,68F05. Key words and phrases. Galois connection, closure operation, interior operation, polarity, axiality. 'Galois' main results were published fourteen years after his early death (at the age of 21 in a duel) by Liouville in his Journal de mathimatiquespures et applqukes (1846). For a translation of Galois' original notes Memoire sur les conditions de rksolubiliti des iquationspar radicaux, see the text by Edwards [ 141. 103 104 ANNALS NEW YORK ACADEMY OF SCIENCES the world of intermediate fields of a field extension E : F and the world of subgroups of the group of automorphisms of E that fix the subfield F (cf. Examples 1 and 19).Today this area is known as Galois theory.Since the proofs of many results in this paper are either well known or easily obtained, we do not include them. Galois connections were originally expressed in a symmetric but contravariant form with transformations that reverse (rather than preserve) order. Early references to this form are [8], [22], [44], and [45]. We use the covariant form since it is more convenient, for example, compositions of Galois connections are handled more easily; it allows for more natural categorical explanations (e.g., by means of adjunctions); and it is more applicable to computer science situations (where relative information preservation is important). For references to the covariant form, see [7], [51], [9], [32], [25], [35], [15], [42], [31], and [19].
THE DEFINITION AND SOME OF ITS CONSEQUENCESWe formulate all our results in terms of partially ordered sets (orposets), that is, sets equipped with a reflexive, transitive, and antisymmetric relation. Everything can easily be generalized to preordered sets (i.e., one may drop the antisymmetry requirement) and even to preordered classes. Applications of these generalizations can be found in Examples 10, 22, and 23, and in [lo]. For a poset 9 = (P, s ) , the order-theoretical dual (P, 2 ) is denoted by 9 OP. =* DEFINITION 1: Consider posets 9 = (P, I) and d = (Q, E). If P -+ Q and Q 2 P are functions such that for allp E P and all q E Q p s a * ( q ) iff a * @ )~q (1)then the quadruple IT = (9, IT *, IT*, d ) is called a Galois connection. We also write 9 5 d (or sometimes just ( I T * , T*)) for the whole Gal...
