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arXiv 2301.08124math.LOcs.LO
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几乎可计算的实数

Nearly Computable Real Numbers

Peter Hertling, Philip Janicki

摘要:

本文称度量空间中的元素序列$(a_n)_n$为近可计算柯西,如果对每个严格递增的可计算函数$r:\mathbb{N}\to\mathbb{N}$,序列$(d(a_{r(n+1)},a_{r(n)}))_n$可计算地收敛到$0$ .我们证明了存在一个严格递增的有理数序列,它几乎是可计算柯西的且是无界的。然后我们称一个实数$α$是近可计算的,如果存在一个可计算的有理数序列$(a_n)_n$,它收敛于$α$并且是近可计算的柯西。很明显,每一个可计算实数都是近似可计算的,并且从Downey和LaForte(2002)的一个结果可以得出,存在一个近似可计算和左可计算的数是不可计算的。我们观察到几乎可计算实数集是一个实闭域,并且在开域的可计算实函数下是闭的,但在任意可计算实函数下不是闭的.除此之外,我们通过证明任何不可计算的几乎可计算实数都是弱$1$ -泛型来加强Hoyrup(2017)和Stephan和Wu(2005)的结果(因此,超免疫而不是Martin-Löf随机)和强Kurtz随机(因此,不是$K$ -平凡的),我们通过证明没有及时单纯集可以图灵约简为几乎可计算的实数来加强Downey和LaForte(2002)的结果。

英文摘要:

In this article we call a sequence $(a_n)_n$ of elements of a metric space nearly computably Cauchy if for every strictly increasing computable function $r:\mathbb{N}\to\mathbb{N}$ the sequence $(d(a_{r(n+1)},a_{r(n)}))_n$ converges computably to $0$. We show that there exists a strictly increasing sequence of rational numbers that is nearly computably Cauchy and unbounded. Then we call a real number $α$ nearly computable if there exists a computable sequence $(a_n)_n$ of rational numbers that converges to $α$ and is nearly computably Cauchy. It is clear that every computable real number is nearly computable, and it follows from a result by Downey and LaForte (2002) that there exists a nearly computable and left-computable number that is not computable. We observe that the set of nearly computable real numbers is a real closed field and closed under computable real functions with open domain, but not closed under arbitrary computable real functions. Among other things we strengthen results by Hoyrup (2017) and by Stephan and Wu (2005) by showing that any nearly computable real number that is not computable is weakly $1$-generic (and, therefore, hyperimmune and not Martin-Löf random) and strongly Kurtz random (and, therefore, not $K$-trivial), and we strengthen a result by Downey and LaForte (2002) by showing that no promptly simple set can be Turing reducible to a nearly computable real number.

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