Classical Descriptive Set Theory

Descriptive set theory studies those subsets of topological spaces (called pointsets) which can be defined, by means of a list of specified operations including, e.g., complement, countable union and intersection, projection, beginning with open sets of the space. Classical descriptive set theory (DST) considers mainly sets in Polish (that is, separable metric) spaces, this is why we shall identify it here as Polish descriptive set theory.

Descriptive Set Theory: Projective Sets

Determinacy of infinite two-player games is a topic of descriptive set theory that has triggered intensive research in theoretical computer science since 1957 when A. Church formulated his "synthesis problem" (regarding the construction of circuits with infinite behavior from logical specifications). In the first part of the lecture we review the fascinating development of the algorithmic theory of infinite games that was started by Church's problem, that enriched automata theory and related fields, and that led to interesting applications in verification and program synthesis. In the second part we turn to the question how to lift this theory from the case of the Cantor space (where a play is a sequence of bits) to the case of the Baire space (where a play is a sequence of natural numbers). While this step does not involve difficulties in classical descriptive set theory, the algorithmic approach raises non-trivial questions since it requires to consider automata that work over infinite alphabets. We present recent results (joint work with B. Brütsch) that provide a solution of Church's synthesis problem in this context, and we point to numerous questions that are still open.

We show that, assuming the Axiom of Determinacy, every non-selfdual Wadge class can be constructed by starting with those of level $\omega _1$ (that is, the ones that are closed under Borel preimages) and iteratively applying the operations of expansion and separated differences. The proof is essentially due to Louveau, and it yields at the same time a new proof of a theorem of Van Wesep (namely, that every non-selfdual Wadge class can be expressed as the result of a Hausdorff operation applied to the open sets). The exposition is self-contained, except for facts from classical descriptive set theory.

Mathematical theories of uncertainty in which measures of uncertainty are now well established are fuzzy set theory, evidence theory, and possibility theory, in addition to classical set theory and probability theory. Some of these theories are connected. Classical set theory is subsumed under fuzzy set theory. Probability theory and possibility theory are branches of evidence theory, while evidence theory is, in turn, a branch of fuzzy measure theory. Although information aspects of fuzzy measure theory have not been investigated as yet, the theory is briefly introduced in this chapter because it represents a broad framework for future research.

During the 1989 European ASL Summer Meeting in Berlin, the authors gave a series of eight lectures (short course) on the topic of the title. This survey article consists basically of the lecture notes for that course distributed to the participants of that conference. We have purposely tried in this printed version to preserve the informal style of the original notes.Let us say first a few things aboui the content of these lectures. Our aim has been to present some recent work in descriptive set theory and its applications to an area of harmonic analysis. Typical uses of descriptive set theory in analysis are most often through regularity properties of definable sets, like measurability, the property of Baire, capacitability, etc., which are used to show that certain problems have solutions that behave nicely. In the theory we will present, definability itself, in fact the precise analysis of the “definable complexity” of certain sets, will be the main concern. It will be through such knowledge that we will be able to infer important structural properties of various objects which will then be used to solve analysis problems.The first lecture provides a short historical introduction to the subject of uniqueness for trigonometric series, which is the area of harmonic analysis whose problems are the origin of this work. As is well known, it was Cantor who proved the first major result in this subject in 1870, and it was his subsequent work here that led him to the creation of set theory.

Labyrinth of Thought discusses the emergence and development of set theory and the set-theoretic approach to mathematics during the period 1850-1940. Rather than focusing on the pivotal figure of Georg Cantor, it analyzes his work and the emergence of transfinite set theory within the broader context of the rise of modern mathematics. The text has a tripartite structure. Part 1, The Emergence of Sets within Mathematics, surveys the initial motivations for a mathematical notion of a set within several branches of the discipline (geometry, algebra, algebraic number theory, real and complex analysis), emphasizing the role played by Riemann in fostering acceptance of the set-theoretic approach. In Part 2, Entering the Labyrinth, attention turns to the earliest theories of sets, their evolution, and their reception by the mathematical community; prominent are the epoch-making contributions of Cantor and Dedekind, and the complex interactions between them. Part 3, In Search of an Axiom System, studies the four-decade period from the discovery of set-theoretic paradoxes to Godel’s independence results, an era during which set theory gradually became assimilated into mainstream mathematics; particular attention is given to the interactions between axiomatic set theory and modern systems of formal logic, especially the interplay between set theory and type theory. A new Epilogue for this second edition offers further reflections on the foundations of set theory, including the dichotomy conception and the well-known iterative conception. The author paints on a grand scale. He sees clearly, and he sees whole. The result is a spacious canvas full of intriguing scenes and portraits from the history of set theory, seamlessly juxtaposed to form a fascinating and accurate picture of a vast area of modern mathematics. It is a must-have book for anyone who wishes to gain a balanced picture of this history. It is written in clear and elegant language for the learner, while experts in the area will enjoy seeing this beautiful presentation of what they already know, perhaps arguing about some of the author’s conclusions and choices of material. (Roger Cooke, University of Vermont)

Local rough set: A solution to rough data analysis in big data

Assuming the Axiom of Constructibility, necessary and sufficient conditions are given for the vanishing of Ext\ for rings A of global dimension 1.Using Martin's Axiom, the necessity of these conditions is shown not to be a theorem of ZFC.Applications are given to abelian group theory, including a partial solution (assuming V = L) to a problem of Baer on the splitting of abelian groups.Some independence results in abelian group theory are also proved.Introduction.In a remarkable recent paper [22], S. Shelah proved that Whitehead's problem is unsolvable in ordinary set theory (Zermelo-Frankel set theory with the Axiom of Choice).More precisely, he proved that two different answers to the problem are obtained when set theory is extended by adding two different axioms: the Axiom of Constructibility and Martin's Axiom.(See Theorem 4.1.)Shelah's arguments were specific to Whitehead's problem and nonhomological in character.In this paper we extend Shelah's techniques in order to obtain some general results about the vanishing of ExtA (for modules over a ring A of global dimension 1) in different models of set theory, and we apply them to some specific problems in abelian group theory.Assuming the Axiom of Constructibility we obtain necessary and sufficient conditions for the vanishing of ExtA (Theorems 1.2 and 1.5).To state the result in.a special case: if C and A are abelian groups of cardinality N,, Ext(C, A) -0 if and only if C is the union of an increasing chain of countable subgroups [Cv\v < «,} such that the chain is smooth (i.e., C0 -U"<oC, for aii iimit ordinals a) and satisfies Ext(C0, A) = 0 and E\i(Cv+x/Cv,A) = 0 for all v < K,.Although the sufficiency of this condition is an easy consequence of classical homological algebra (see Theorem 1.2), the proof of necessity makes use of the Axiom of Constructibility (Theorem 1.5).The necessity of this condition is not provable in ordinary set theory; in particular the condition is not necessary when Martin's Axiom is assumed ( §3).

Rough function is one of interesting new directions of the classical Pawlak rough set theory. This paper presents a generalized rough function with one variable under the framework of classical Pawlak rough set theory. In the definition of generalized rough function, we extend the domain of the classical rough function from the general natural number set to integer set, and then the rough function with one variable defined on an integer set is obtained. After that, we investigate the basic properties of the proposed rough function with one variable base on the classical function analysis theory in detail. Furthermore, some interesting results about the rough function with one variable over integer set are given and the relationship between the existing literatures is discussed. The contribution of this is twofold: one is present a new concept of rough function with one variable over the integer set. Another is of enriches the basis of theory of classical Pawlak rough set and also extends the areas of the applications.

This chapter is an overview of current research regarding measures of uncertainty and information in five mathematical theories: classical set theory, fuzzy set theory, probability theory, possibility theory, and the Dempster-Shafer theory of evidence. Three types of uncertainty are involved: nonspecificity, entropy-like uncertainty, and fuzziness.

Tarski's theory of definability: common themes in descriptive set theory, recursive function theory, classical pure logic, and finite-universe logic

Partial functions can be easily represented in set theory as certain sets of ordered pairs. However, classical set theory provides no special machinery for reasoning about partial functions. For instance, there is no direct way of handling the application of a function to an argument outside its domain as in partial logic. There is also no utilization of lambda-notation and sorts or types as in type theory. This paper introduces a version of von-Neumann-Bernays-Godel set theory for reasoning about sets, proper classes, and partial functions represented as classes of ordered pairs. The underlying logic of the system is a partial first-order logic, so class-valued terms may be nondenoting. Functions can be specified using lambda-notation, and reasoning about the application of functions to arguments is facilitated using sorts similar to those employed in the logic of the IMPS Interactive Mathematical Proof System. The set theory is intended to serve as a foundation for mechanized mathematics systems.

In [3] it was proved that every convex Borel (= Baire) set in a finite dimensional real Banach space can be obtained, starting from the closed (or compact) convex sets, by the iteration of countable increasing unions and countable decreasing intersections.In §2 of this note we define some concepts of the descriptive theory of convex sets in locally convex spaces. We prove several theorems, which are analogous to the standard theorems of the descriptive theory of sets in topological spaces.

Estimation of uncertainty in knowledge extracted from a repository of data is a challenging issue in artificial intelligence and cognitive sciences. The rough set theory is a prominent technique for data mining from a repository of domain data. A rough set is a formal approximation of a crisp set in terms of a pair of sets with respect to equivalence relations. The concept of lower and upper approximations is based on the assumption that information is associated with every object described by a set of attributes. The classical rough set theory with the equivalence class concept and static dependency cannot handle any uncertainty in attribute level which seems to be a major limitation. Several extensions on classical rough set theory have remarkable contributions in different applications. But excavation of the nature of dependency of the attributes and the estimation of uncertainty is still an unsolved issue. This paper proposes an extension in the classical rough set theory by introducing probabilistic relevancy of the attributes. Mining the correlation between attributes in terms of the probabilistic dependency coefficient is an important way out in discovering the uncertainty. The roughness in contributing knowledge by an attribute can be represented using approximations in the probabilistic dependency space. Using the approximations in the values of the probabilistic dependency coefficient, the inherent uncertainty can be estimated and handled more precisely.