Domains and Probability Measures: A Topological Retrospective

Domain theory has seen success as a semantic model for high-level programming languages, having devised a range of constructs to support various effects that arise in programming. One of the most interesting - and problematic - is probabilistic choice, which traditionally has been modeled using a domain-theoretic rendering of sub-probability measures as valuations. In this talk, I will place the domain-theoretic approach in context, by showing how it relates to the more traditional approaches such as functional analysis and set theory. In particular, we show how the topologies that arise in the classic approaches relate to the domain-theoretic rendering. We also describe some recent developments that extend the domain approach to stochastic process theory

Riemann and Edalat integration on domains

The main result of this paper is that the domain-theoretic approach to the generalized Riemann integral first introduced by Edalat extends to a large class of spaces that can be realized as the set of maximal points of domains. The approach is based on the theory of a Riemann-Stieltjes type integral on a topological space with respect to a finitely additive measure. We develop the theory of this integral for a bounded function f defined on the maximal points of a continuous domain and show that it gives an alternate approach to the Edalat integral. © 2002 Elsevier B.V. All rights reserved

Recursive Solution of Initial Value Problems with Temporal Discretization

We construct a continuous domain for temporal discretization of differential equations. By using this domain, and the domain of Lipschitz maps, we formulate a generalization of the Euler operator, which exhibits second-order convergence. We prove computability of the operator within the framework of effectively given domains. The operator only requires the vector field of the differential equation to be Lipschitz continuous, in contrast to the related operators in the literature which require the vector field to be at least continuously differentiable. Within the same framework, we also analyze temporal discretization and computability of another variant of the Euler operator formulated according to Runge-Kutta theory. We prove that, compared with this variant, the second-order operator that we formulate directly, not only imposes weaker assumptions on the vector field, but also exhibits superior convergence rate. We implement the first-order, second-order, and Runge-Kutta Euler operators using arbitrary-precision interval arithmetic, and report on some experiments. The experiments confirm our theoretical results. In particular, we observe the superior convergence rate of our second-order operator compared with the Runge-Kutta Euler and the common (first-order) Euler operators.Comment: 50 pages, 6 figure

General Riemann Integrals and Their Computation via Domain.

In this work we extend the domain-theoretic approach of the generalized Riemann integral introduced by A. Edalat in 1995. We begin by laying down a related theory of general Riemann integration for bounded real-valued functions on an arbitrary set X with a finitely additive measure on an algebra of subsets of X. Based on the theory developed we obtain a formula to calculate integral of a bounded function in terms of the regular Riemann integral. By classical extension theorems on set functions we can further extend this generalized Riemann integral to more general set functions such as valuations on lattices of subsets. For the setting of bounded functions defined on a continuous domain D with a Borel measure for the Scott topology, we can compute the Riemann integral of a function effectively and so the value of the integral can be obtained up to a given accuracy. By invoking the results of J. Lawson we can extend this type of Riemann integral to maximal point spaces, as a special case of the Polish spaces. Furthermore we show that when X is a compact metric space, our approach of Riemann integration is equivalent to the generalized Riemann integration introduced by A. Edalat in the sense that the two integrals yield the same value. Finally we prove that the approach of integration taken by R. Heckmann is equivalent to our approach, and the values of the integrals are the same

PCF extended with real numbers: a domain-theoretic approach to higher-order exact real number computation

We develop a theory of higher-order exact real number computation based on Scott domain theory. Our main object of investigation is a higher-order functional programming language, Real PCF, which is an extension of PCF with a data type for real numbers and constants for primitive real functions. Real PCF has both operational and denotational semantics, related by a computational adequacy property. In the standard interpretation of Real PCF, types are interpreted as continuous Scott domains. We refer to the domains in the universe of discourse of Real PCF induced by the standard interpretation of types as the real numbers type hierarchy. Sequences are functions defined on natural numbers, and predicates are truth-valued functions. Thus, in the real numbers types hierarchy we have real numbers, functions between real numbers, predicates defined on real numbers, sequences of real numbers, sequences of sequences of real numbers, sequences of functions, functionals mapping sequences to numbers (such as limiting operators), functionals mapping functions to numbers (such as integration and supremum operators), functionals mapping predicates to truth-values (such as existential and universal quantification operators), and so on. As it is well-known, the notion of computability on a domain depends on the choice of an effective presentation. We say that an effective presentation of the real numbers type hierarchy is sound if all Real PCF definable elements and functions are computable with respect to it. The idea is that Real PCF has an effective operational semantics, and therefore the definable elements and functions should be regarded as concretely computable. We then show that there is a unique sound effective presentation of the real numbers type hierarchy, up to equivalence with respect to the induced notion of computability. We can thus say that there is an absolute notion of computability for the real numbers type hierarchy. All computable elements and all computable first-order functions in the real numbers type hierarchy are Real PCF definable. However, as it is the case for PCF, some higher-order computable functions, including an existential quantifier, fail to be definable. If a constant for the existential quantifier (or, equivalently, a computable supremum operator) is added, the computational adequacy property remains true, and Real PCF becomes a computationally complete programming language, in the sense that all computable functions of all orders become definable. We introduce induction principles and recursion schemes for the real numbers domain, which are formally similar to the so-called Peano axioms for natural numbers. These principles and schemes abstractly characterize the real numbers domain up to isomorphism, in the same way as the so-called Peano axioms for natural numbers characterize the natural numbers. On the practical side, they allow us to derive recursive definitions of real functions, which immediately give rise to correct Real PCF programs (by an application of computational adequacy). Also, these principles form the core of the proof of absoluteness of the standard effective presentation of the real numbers type hierarchy, and of the proof of computational completeness of Real PCF. Finally, results on integration in Real PCF consisting of joint work with Abbas Edalat are included