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Specker's theorem, cluster points, and computable quantum functions

Published online by Cambridge University Press:  30 March 2017

Iraj Kalantari
Affiliation:
Western Illinois University
Ali Enayat
Affiliation:
American University, Washington DC
Iraj Kalantari
Affiliation:
Western Illinois University
Mojtaba Moniri
Affiliation:
Tarbiat Modares University, Tehran, Iran
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Summary

Abstract.The present paper continues the work we began in [KalWel95, KalWel96, KalWel03, KalWel05].

Specker [Spe49] proved existence of a computable sequence of computable reals whose limit is not a computable real by using a noncomputable c.e. (computably enumerable) set. He did this by requiring his sequence to evade every computable point as a limit.

We study similar and generalized results in our filter-based approach to computable analysis and computable topology. We strategically construct evading sequences of basic open sets to generalize Specker's work. An evading sequence is a sequence of basic open sets such that any sequence of points with one point chosen to lie in each basic open set has the same cluster points as any other such sequence. This set of cluster points is the set of Specker cluster points of the evading sequence. We devise two methods for obtaining evading sequences such that all the Specker cluster points lie in the spectrum of a given avoidance function. We then use these methods to construct evading sequenceswhere their cluster points can be of one or more types (computable, avoidable, or shadow) and of any possible cardinality for each type. Finally, we use the acquired machinery to reveal two facts of the fine structure of the lower semi-lattice of domains of computable quantum functions under the relation of ‘subset’. Specifically, we show that we cannot always interpolate a computable quantum function between two nested computable quantum functions by constructing a pair of nested computable quantum functions whose domains differ in exactly one point. In contrast, we show that any time two nested computable quantum functions exist whose domains differ by at least two points, we can interpolate another computable quantum function between them.

Introduction.There are three classical approaches to general topology : Fréchet's abstract spaces, Hausdorff's neighborhood classes andKuratowski's closure classes. The fundamental objects with which one works in all these approaches are points. But a point can be viewed as a limit of a sequence of other points or the sole member of an intersection of a nested sequence of open intervals. These approaches to points are often useful.

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