Theory of Computing Systems

, Volume 43, Issue 3–4, pp 603–624 | Cite as

Classification of Computably Approximable Real Numbers

  • Xizhong Zheng


A real number is called computably approximable if it is the limit of a computable sequence of rational numbers. Therefore the complexity of these real numbers can be classified by considering the convergence speed of computable sequences. In this paper we introduce a natural way to measure the convergence speed by counting the number of jumps of given sizes that appear after certain stages. Bounding the number of such kind of big jumps by different bounding functions, we introduce various classes of real numbers with different levels of approximation quality. We discuss further their mathematical properties as well as computability theoretical properties.


Computable real numbers Computably approximable real numbers Hierarchy 


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© Springer Science+Business Media, LLC 2007

Authors and Affiliations

  1. 1.Department of Computer ScienceJiangsu UniversityZhenjiangChina
  2. 2.Theoretische InformatikBTU CottbusCottbusGermany

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