A lattice-theoretical fixpoint theorem and its applications

TLDR: In this paper, the authors formulate and prove an elementary fixpoint theorem which holds in arbitrary complete lattices, and give various applications (and extensions) of this result in the theories of simply ordered sets, real functions, Boolean algebras, as well as in general set theory and topology.

Abstract: 1. A lattice-theoretical fixpoint theorem. In this section we formulate and prove an elementary fixpoint theorem which holds in arbitrary complete lattices. In the following sections we give various applications (and extensions) of this result in the theories of simply ordered sets, real functions, Boolean algebras, as well as in general set theory and topology. * By a lattice we understand as usual a system 21 = (A 9 <) formed by a non-empty set A and a binary relation <; it is assumed that < establishes a partial order in A and that for any two elements a f b E A there is a least upper bound (join) a u b and a greatest lower bound (meet) an b. The relations >L, <, and > are defined in the usual way in terms of <. The lattice 21 = (A, <) is called complete if every subset B of A has a least upper bound ΌB and a greatest lower bound Πβ. Such a lattice has in particular two elements 0 and 1 defined by the formulas 0 = ΓU and 1 = 11,4. Given any two elements a 9 b E A with a < b, we denote by [a 9 b] the interval with the endpoints a and b, that is, the set of all elements x E A for which a < x < b; in symbols, [ a,b] = E x [x E A and a .< x .< b ]. The system \[α,6], <) is clearly a lattice; it is a complete if 21 is complete. We shall consider functions on A to A and, more generally, on a subset B of A to another subset C of A. Such a function / is called increasing if, for any 1 For notions and facts concerning lattices, simply ordered systems, and Boolean algebras consult [l].