# Tag Info

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I work in real-number computation, and I wish I knew the real answer. But I can speculate. It's a sociological problem, I think. The community of people who work on exact real arithmetic consists of theoreticians who are not used to developing software. So they usually relegate the task of implementation to students (a notable exception is Norbert Müller's ...

16

This is not a research-level question, but since the general level of interest seems high, here is an answer. I cannot guess from your question whether you're shooting for something that will result in the usual computable numbers, or you're trying to surpass that. First we have Turing's definition of computable real number, and it is the one others have ...

13

I am not exactly sure what the question is here, but I can try to say a bit to clean up possible misunderstandings. First of all, if we are talking about complexity of a map $f : \mathbb{R} \to \mathbb{R}$, it makes no sense to ask "What is a good representation for $\sqrt{2}$?" Instead, you have to ask "What is a good representation for all inputs of $f$?"....

10

In general, people always care about floating point errors. However I disagree with Andrej, and I do not think that floats are preferred to arbitrary precision reals (for the most part) because of sociological reasons. I believe the main argument against exact computation of reals is one of performance. So the short answer is, whenever performance is more ...

7

No, it is not decidable. A good heuristic to answer such questions is the following: every computable map is continuous. If you could decide whether $f(x) = 0$ for all $x \in \mathbb{C}$, then the characteristic map $d$ of such a decision procedure, namely $$d : f \mapsto \begin{cases} 1 & \text{if \forall x \in \mathbb{C} . f(x) = 0}\\ 0 & \text{... 4 Computing an 'infinite' object is usually defined based on Turing machines with one-way output tape; cmp. Section 2.1 in [Weihrauch'00]. This also asserts closure under composition. [Turing'36] first defined computability of real numbers based on their binary (historically actually: decimal) and in [Turing'37] corrected himself to consider sequences of ... 3 How about, x\in[0,1] is computable if there is a TM M which, on input n\in\mathbb{N}, prints the first n digits of the decimal expansion of x and then halts. Edit (13-Jul-2021). This definition of computable real numbers, though natural (apparently, it's the one Turing gave), has problems. It's better to demand that on input n, the TM prints a ... 3 The standard solution is to require write only tape if you want to use TTE. Obviously you can have RW work tapes. Another solution is information theoretic (domain theory) and says that you get better and better approximations approximation to the real number as internals and the limit is a single number. Also keep in mind that decimal representation of ... 3 To turn my comment into an answer... Andrej Bauer in this post makes the parenthetical claim An important theorem states that any two representations of reals which are acceptable are actually computably isomorphic. He never explicitly defines acceptable or cites the theorem. I assume the OPs question is the following. What theorem is Andrej Bauer ... 3 No, it is undecidable. Imagine a TM that outputs a sequence 0.1111\ldots that may be finite or not. If it is finite, the conversion algorithm should give some fraction like \frac{11\ldots11}{10\ldots000}. If it is infinite (the TM doesn't halt) then the output should be \frac{1}{9}. Any program that converts between the TM representation of a number ... 3 Another model possibly to explore, is that of the Feasible RAM model. This is a modified real RAM model for Real computation, Feasible RAM, or a modified RAM model which uses both the discrete, and real valued arithmetic operations. This model allows for real, and discrete operations, and the Turing model, is interchangeable with it. The Feasible RAM model ... 2 The first theorem of the form you are asking about was proved by Y. Moschovakis in Notation systems and recursive ordered fields, Compositio Mathematica 17:40–71 (1965). Then in the context of Type Two Effectivity a similar theorem was proved by P. Hertling, see A real number structure that is effectively categorical, Mathematical Logic Quarterly, 45(2):147–... 2 One solution is what Radu GRIGore suggested, namely requiring that every digit becomes fixed after some (finite) number of steps. Of course, this comes with the practical issue that you never know whether a digit is already fixed. For defining computability, this is seems to be fine since the Turing machine is never actually used in practice by someone who ... 1 I guess I have an alternative solution. Let f :\subseteq \Sigma^{\omega} \to \Sigma^{\omega} be computable. Define$$ h(u) = v \mbox{ iff } f(u\Sigma^{\omega}) = \{v\} \mbox{ with $u$ minimal}  and let $h(u)$ diverge otherwise. Then $\mbox{dom}(h)$ is prefix-free for if $h(u) = v$ then $f(u\Sigma^{\omega}) = \{v\}$ as the machine behaves the same when ...

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