Let S2S [WS2S] respectively be the strong [weak] monadic second order theory of the binary tree T in the language of two successor functions. An S2S-formula whose free variables are just individual variables defines a relation on T (rather than on the power set of T). We show that S2S and WS2S define the same relations on T, and we give a simple characterization of these relations.

If α is a singular cardinal (either real or fake) in L, I exhibit many natural α-re subsets, defined uniformly from the ⊿1 subsets of a. If α. is a true cardinal this provides an uppersemilattice (usl) embedding from the lattice of ⊿1 subsets of α into the usl of α-re-degrees. It will also be shown that this embedding cannot be extended to the Σ1 subsets of α.

Kantorovich and Livenson [6] initiated the study of infinitary Boolean operations applied to the subsets of the Baire space and related spaces. It turns out that a number of interesting collections of subsets of the Baire space, such as the collection of Borel sets of a given type (e.g. the Fσ sets) or the collection of analytic sets, can be expressed as the range of an ω-ary Boolean operation applied to all possible ω-sequences of clopen sets. (Such collections are called clopen-ω-Boolean.) More recently, the ranges of I-ary Boolean operations for uncountable I have been considered; specific questions include whether the collection of Borel sets, or the collection of sets at finite levels in the Borel hierarchy, is clopen-I-Boolean.

The main purpose of this paper is to give a characterization of those collections of subsets of the Baire space (or similar spaces) that are clopen-I-Boolean for some I. The Baire space version can be stated as follows: a collection of subsets of the Baire space is clopen-I-Boolean for some I iff it is nonempty and closed downward and σ-directed upward under Wadge reducibility, and in this case we may take I = ω2. The basic method of proof is to use discrete subsets of spaces of the form K2 to put a number of smaller clopen-I-Boolean classes together to form a large one. The final section of the paper gives converse results indicating that, at least in some cases, ω2 cannot be replaced by a smaller index set.

The lambda calculus was conceived by Church, and to some extent by Curry (see [1] and [2]), as a general theory of functions, more precisely as a formalization of two basic functional concepts: application and abstraction. This claim to universality was impossible to maintain, for it soon became apparent that functions in the calculus are required to have properties which are by no means universal. Self-application is certainly a very special feature, but more relevant from our point of view is the existence of fixed points, which is definitely a nonuniversal property of functions. This situation was not ignored, but was considered rather as a form of inconsistency very much related to the paradoxes of set theory (for example, see the discussion of the paradoxical combinator in [2]).

More recently, the lambda calculus has been studied by people in computer science, and as a result the algorithmic nature of the system has been more explicitly recognized. Eventually, the right approach was taken by D. Scott (see [4] and [5]) who restricted the objects of the calculus to continuous functionals over complete lattices, which can be combined in structures where application and abstraction are properly interpreted. It soon became clear that complete lattices were not necessary, and more general structures were introduced. On the other hand the requirement of continuity was essential for the construction, and has remained a basic feature. It is one of the purposes of this paper to show that in the general situation continuity can be replaced by monotonicity.

The work of Scott was extended by Wadsworth [6], who gave a precise formulation of the operational semantics, and proved the equivalence to Scott's interpretation.