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Computability
AI
The paper presents an investigation into the computability of real functions through two approaches, one rooted in Grzegorczyk's framework and another by Tent and Ziegler. It establishes the equivalence of these approaches under certain general conditions, thus broadening the understanding of uniform computability and acceptable pairs of functions and operators. Definitions regarding the naming of real numbers and computing systems for real functions are provided, alongside a characterization theorem affirming the conditions for a function to be considered uniformly computable.
Lecture Notes in Computer Science, 2011
▸ For any class F of total functions in N, we define what it means for a real function to be conditionally F-computable. This notion extends the notion of uniform F-computability of real functions introduced in the paper [SkWeGe 10].
Annals of Pure and Applied Logic, 2013
For any class of operators which transform unary total functions in the set of natural numbers into functions of the same kind, we define what it means for a real function to be uniformly computable or conditionally computable with respect to this class. These two computability notions are natural generalizations of certain notions introduced in a previous paper co-authored by Andreas Weiermann and in another previous paper by the same authors, respectively. Under certain weak assumptions about the class in question, we show that conditional computability is preserved by substitution, that all conditionally computable real functions are locally uniformly computable, and that the ones with compact domains are uniformly computable. The introduced notions have some similarity with the uniform computability and its non-uniform extension considered by Katrin Tent and Martin Ziegler, however, there are also essential differences between the conditional computability and the non-uniform computability in question.
2018
The TTE-approach to computability of real functions uses infinitary names of the argument’s and the function’s values, computability being defined as the existence of some algorithmic procedure transforming the names of any argument’s value into ones of the corresponding value of the function. Two ways to avoid using such names are considered in the present paper. At each of them, the corresponding characterization of computability of real functions is through the existence of an appropriate recursively enumerable set establishing some relation between rational approximations of the argument’s value and rational approximations of the corresponding value of the function. The characterizations in question are derived from ones for computability of functions in metric and in topological spaces.
2011
Computable analysis is an approach to real continuous computation that is based on extending the normal Turing machine model. It was introduced by A. Turing 1936, A. Grzegorczyk 1955, and D. Lacombe 1955. Since the introduction of Moore's real recursion theory in 1996 several classes of computable analysis functions have been characterized by functions algebras. On the one hand these algebraic characterizations provide a unifying theoretical framework that interconnects computable analysis with other approaches to real computation such as the GPAC and Moore's recursion theory. On the other hand they provide machine-independent characterizations and hence a different perspective on computable analysis, a perspective that is more intuitive and natural especially from the vantage point of the mathematical analysis community. In this article we give an introduction to the field of computable analysis and a survey of the different algebraic characterizations of computable analysis classes starting from the elementary functions up to the total computable ones passing through the Grzegorczyk hierarchy. Unfortunately, not much work has been done in characterizing the sub elementary, in particular the lower complexity-theoretic classes. Some of the author's published work in that latter direction are presented in this article. This includes the introduction of a function algebra that is an extension of the Bellantoni-Cook class. The extended class can exactly characterize discrete polynomial time computation, however, can only partially characterize polynomial time real computation. Furthermore, there exists a gap between the computation concept over the rational numbers and the corresponding one over the reals. This difference is illustrated by the existence of computable rational functions whose extension to the reals are not computable and vice versa. Understanding this gap might help us extend the algebraic discrete complexity classes to the reals. This article surveys many of the major results in the area and their implications.
Domains and Processes, 2001
There have been many suggestions for what should be a computable real number or function. Some of them exhibited pathological properties. At present, research concentrates either on an application of Weihrauch's Type Two Theory of Effectivity or on domain-theoretic approaches, in which case the partial objects appearing during computations are made explicit. A further, more analysis-oriented line of research is based on Grzegorczyk's work. All these approaches are claimed to be equivalent, but not in all cases proofs have been given. In this paper it is shown that a real number as well as a real-valued function are computable in Weihrauch's sense if and only if they are definable in Escardó's functional language Real PCF, an extension of the language PCF by a new ground type for (total and partial) real numbers. This is exactly the case if the number is a computable element in the continuous domain of all compact real intervals and/or the function has a computable extension to this domain. For defining the semantics of the language Real PCF a full subcategory of the category of bounded-complete ω-continuous directed-complete partial orders is introduced and it is defined when a domain in this category is effectively given. The subcategory of effectively given domains contains the interval domain and is Cartesian closed. * The paper mainly contains results from the second author's diploma thesis [18] written under the supervision of the first author.
Theoretical Computer Science, 2004
This is an investigation into exact real number computation using the incremental approach of Potts (Ph.D. where numbers are represented as inÿnite streams of digits, each of which is a M obius transformation. The objective is to determine for each particular system of digits which functions R → R can be computed by a ÿnite transducer and ultimately to search for the most ÿnitely expressible M obius representations of real numbers. The main result is that locally such functions are either not continuously di erentiable or equal to some M obius transformation. This is proved using elementary properties of ÿnite transition graphs and M obius transformations. Applying the results to the standard signed-digit representations, we can classify functions that are ÿnitely computable in such a representation and are continuously di erentiable everywhere except for ÿnitely many points. They are exactly those functions whose graph is a fractured line connecting ÿnitely many points with rational coordinates.
Journal of Logic and Computation, 2010
The results on the subject of the talk are obtained by the authors and Ivan Georgiev during the period June 2008-July 2009. Outline 1 Introduction The class M 2 F-computability of real numbers 2 Proving M 2-computability by using appropriate partial sums M 2-computability of the number e M 2-computability of Liouville's number A partial generalization 3 Stronger tools for proving M 2-computability of real numbers M 2-computable real-valued function with natural arguments Logarithmically bounded summation M 2-computability of sums of series 4 Applications of the stronger tools M 2-computability of π A generalization Some other M 2-computable constants Preservation of M 2-computability by certain functions 5 Conclusion 6 References
We try to classify many-valued functions of type Á ½ ¢ ¡ ¡ ¡ ¢ Á Ò Á ¼ (for some compact intervals Á Ê) that can be exactly realized by a finite transducer using some stream representation of the real numbers. We restrict our study to the so-called IFS-representations where numbers are represented by infinite compositions of contracting functions on a compact interval. IFS-representations include the radix (e.g. decimal, signed binary) representations and representations based on Möbius transformations (=LFT's).
Theoretical Computer Science, 2005
We present an analog and machine-independent algebraic characterization of elementarily computable functions over the real numbers in the sense of recursive analysis: we prove that they correspond to the smallest class of functions that contains some basic functions, and closed by composition, linear integration, and a simple limit schema.
Bulletin of the American Mathematical Society, 1989
We present a model for computation over the reals or an arbitrary (ordered) ring R. In this general setting, we obtain universal machines, partial recursive functions, as well as JVP-complete problems. While our theory reflects the classical over Z (e.g., the computable functions are the recursive functions) it also reflects the special mathematical character of the underlying ring R (e.g., complements of Julia sets provide natural examples of R. E. undecidable sets over the reals) and provides a natural setting for studying foundational issues concerning algorithms in numerical analysis.
Archive for Mathematical Logic, 2008
The basic motivation behind this work is to tie together various computational complexity classes, whether over different domains such as the naturals or the reals, or whether defined in different manners, via function algebras (Real Recursive Functions) or via Turing Machines (Computable Analysis). We provide general tools for investigating these issues, using two techniques we call approximation and lifting. We use these methods to obtain two main theorems. First we provide an alternative proof of the result from Campagnolo, Moore and Costa [3], which precisely relates the Kalmar elementary computable functions to a function algebra over the reals. Secondly, we build on that result to extend a result of Bournez and Hainry [1], which provided a function algebra for the C 2 real elementary computable functions; our result does not require the restriction to C 2 functions. In addition to the extension, we provide an alternative approach to the proof. Their proof involves simulating the operation of a Turing Machine using a function algebra. We avoid this simulation, using a technique we call lifting, which allows us to lift the classic result regarding the elementary computable functions to a result on the reals. The two new techniques bring a different perspective to these problems, and furthermore appear more easily applicable to other problems of this sort.
lix.polytechnique.fr
Abstract. Several algebraic machine-independant characterizations of computable functions over the reals have been obtained recently. In par-ticular nice connections between the class of computable functions (and some of its sub and sup-classes) over the reals and ...
Proc. of the Thirty-Second Annual ACM/IEEE Symposium on Logic in Computer Science (LICS 2017), 2017
—We investigate interrelationships among different notions from mathematical analysis, effective topology, and classical computability theory. Our main object of study is the class of computable functions defined over a bounded domain with the boundary being a left-c.e. number. We investigate necessary and sufficient conditions under which such function can be computably extended. It turns out that this depends on the behavior of the function near the boundary as well as on the class of left-c.e. numbers to which the boundary belongs, that is, how it can be constructed. Of particular interest a class of functions is investigated: sawtooth functions constructed from computable enumerations of c.e. sets.
Proceedings of the first annual ACM symposium on Theory of computing - STOC '69, 1969
The structure of the functions computable in time or space bounded by t is investigated for recursive functions t. The t-computable classes are shown to be closed under increasing recursively enumerable unions; as a corollary the primitive recursive functions are shown to equal the t-computable functions for a certain recursive t. Any countable partial order can be isomorphically embedded in the family of t-computable classes partially ordered by set inclusion. For any recursive t, there is a recursive t' which is (approximately) equal to an actual running time such that the t-computable functions equal the t'-computable functions.
Foundations of Computer Science, …, 1988
On a Theory of Computation over the Real Numbers; NP Completeness, Recursive Functions and Universal Machines* (Extended Abstract) Lenore Blum *'1 Department of Mathematics and Computer Science, Mills College Oakland, CA 94613 and Department of Mathematics ...
This definition formalises the notion of A being reducible to B if and only if there is a procedure such that given any enumeration of the members of B the procedure uniformly provides us with an enumeration of A. We notice that an e-operator can be thought of as being the union of a r.e. set of T-operators.
Recursive analysis is the most classical approach to model and discuss computations over the reals. It is usually presented using Type 2 or higher order Turing machines. Recently, it has been shown that computability classes of functions computable in recursive analysis can also be defined (or characterized) in an algebraic machine independent way, without resorting to Turing machines. In particular nice connections between the class of computable functions (and some of its sub-and sup-classes) over the reals and algebraically defined (sub-and sup-) classes of R-recursive functionsà la Moore 96 have been obtained. However, until now, this has been done only at the computability level, and not at the complexity level. In this paper we provide a framework that allows us to dive into the complexity level of functions over the reals. In particular we provide the first algebraic characterization of polynomial time computable functions over the reals. This framework opens the field of implicit complexity of functions over the reals, and also provide a new reading of some of the existing characterizations at the computability level.
Mathematical Logic Quarterly, 1997
Iterative characterizations of computable unary functions are useful patterns for the definition of programming languages based on iterative constructs. The features of such a characterization depend on the pairing producing it: this paper offers an infinite class of pairings involving very nice features.
Lecture Notes in Computer Science, 2004
We present an analog and machine-independent algebraic characterization of elementarily computable functions over the real numbers in the sense of recursive analysis: we prove that they correspond to the smallest class of functions that contains some basic functions, and closed by composition, linear integration, and a simple limit schema. We generalize this result to all higher levels of the Grzegorczyk Hierarchy. Concerning recursive analysis, our results provide machine-independent characterizations of natural classes of computable functions over the real numbers, allowing to define these classes without usual considerations on higher-order (type 2) Turing machines. Concerning analog models, our results provide a characterization of the power of a natural class of analog models over the real numbers.
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