Published online by Cambridge University Press: 20 November 2018
Unless further restricted, P, Q, and R denote arbitrary partially ordered sets whose order relations are all written “≦” .
An isotone mapping ϕ: P → Q is said to be residuated if there is an isotone mapping ψ: Q → P such that
(RM 1) xϕψ ≧ x for all x i n P;
(RM 2) yψϕ ≦ for all y in Q.
Let Q* denote the partially ordered set with order relation dual to that of Q.
(A) The following conditions are equivalent:
(i) ϕ: P → Q* is a Galois connection;
(ii) ϕ: P → Q is a residuated mapping;
(iii) Max{z ∈ P: zy ≦ y} exists for all y in Q and is equal to yψ.
Since ψ is uniquely determined by ϕ, it will be denoted by ϕ+.