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There is a claim on https://en.wikipedia.org/wiki/Super-recursive_algorithm#Inductive_Turing_machines that 'Simple inductive Turing machines are equivalent to other models of computation such as general Turing machines of Schmidhuber, trial and error predicates of Hilary Putnam, limiting partial recursive functions of Gold, and trial-and-error machines of Hintikka and Mutanen.[1] More advanced inductive Turing machines are much more powerful. There are hierarchies of inductive Turing machines that can decide membership in arbitrary sets of the arithmetical hierarchy(Burgin 2005). In comparison with other equivalent models of computation, simple inductive Turing machines and general Turing machines give direct constructions of computing automata that are thoroughly grounded in physical machines'.

Wiki also says these are different from infinite time Turing machines.

Are inductive turing machines different from turing machines with oracles?

Are inductive turing machines physically realizable (at least in the same sense of realizaility of Turing machines as Intel processors with bounded RAM and one that degrades over time)?

Can inductive turing machines solve the halting problem (essentially Hilbert's $10$th as well)?

Please refer here for overcoming Church-Turing Hypothesis with Inductive Turing machines https://en.wikipedia.org/wiki/Super-recursive_algorithm#Relation_to_the_Church.E2.80.93Turing_thesis.

Here is another article (published in communications of the ACM and well cited) http://www.columbia.edu/itc/hs/medinfo/g6080/misc/p82-burgin.pdf.

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    $\begingroup$ Computability theory is not really about what is "physically realizable." Are Turing machines themselves physically realizable? No, because once the paper tape becomes large enough, it will have to be in orbit somehow and will inevitably tear; or if it is organized compactly somehow, then it will collapse into a stellar mass or black hole from its own gravity. It seems that there will be a finite physical upper bound for the size of a Turing machine that can actually operate, and so the physically operable Turing machines do not constitute a Turing-complete model of computation. $\endgroup$ Commented Jul 28, 2017 at 23:32
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    $\begingroup$ Not to mention the further complicating issue that according to some physical theories, the physical universe has finite size, matter and energy. $\endgroup$ Commented Jul 28, 2017 at 23:47
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    $\begingroup$ @JoelDavidHamkins Or thermodynamics - Turing machines aren't supposed to degrade/fall apart! (And of course black hole computing is similarly rough on the hardware.) $\endgroup$ Commented Jul 29, 2017 at 1:10
  • $\begingroup$ @JoelDavidHamkins fair enough but Burgin claims inductive machines are physically realizable in every way that we all think our computers are Turing machines and so the query 'can the Hilbert 10th problem be solved by these machines?' is still valid provided if there is any reason at all to believe these machines differ from Turing machines with oracles (which are in no way realizable in sense I just mentioned). Refer comment here by Burgin blog.computationalcomplexity.org/2007/03/… starting from 'Dear Lance,'. Note DARPA funds superrecursive algorithms. $\endgroup$
    – Turbo
    Commented Jul 29, 2017 at 1:23
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    $\begingroup$ If an 'inductive Turing Machine' cannot say when it has obtained its result, how does one know what the result of the computation is? (Note that there's some commentary on the talk page of the linked Wikipedia article that asks the same question, with no real resolution.) $\endgroup$ Commented Jul 29, 2017 at 3:46

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Let me try to answer the actual question that was asked. The Wikipedia page defines inductive Turing machines as follows:

An inductive Turing machine is a definite list of well-defined instructions for completing a task which, when given an initial state, will proceed through a well-defined series of successive states, eventually giving the final result. The difference between an inductive Turing machine and an ordinary Turing machine is that an ordinary Turing machine must stop when it has obtained its result, while in some cases an inductive Turing machine can continue to compute after obtaining the result, without stopping.

Two remarks.

  1. I assume that when the description says "eventually giving the final result," what is meant is that there is a stage after which the computation is always displaying that result as output. This makes the concept identical to what has also been known by the term computability-in-the-limit, as well as other terminology. One naturally extends the concept to partial functions, by insisting that for inputs not in the domain, what we want is for the outputs not to converge or stabilize. This is evidently the simple model of inductive machine; the wikipedia page makes references to a hierarchy of more powerful machines.

  2. Although the Wikipedia page makes numerous references to Mark Burgin — his name appears 24 times in the linked article — to my understanding of the history of the subject, the particular concept of computability-in-the-limit has been well understood and analyzed by computability theorists much earlier than Burgin's writings.

To my way of thinking, the main thing to say about this notion of computability is the following, which is commonly given as an exercise in computability theory courses.

Theorem. For any function $f$, the following are equivalent.

  1. $f$ is computable by an inductive Turing machine; that is, $f$ is computable in the limit.

  2. $f$ is computable (in the usual sense) by a Turing machine equipped with an oracle for the halting problem.

  3. The graph of $f$ is $\Sigma_2$-definable.

Proof. ($1\to 3$). If $f$ is computable by an inductive Turing machine, then $f(a)=b$ if and only if there is some stage of the inductive computation on input $a$ such that at any later stage, the output is still $b$. This is a $\Sigma_2$ definition of the graph of $f$.

($3\to 2$) If the graph of $f$ is $\Sigma_2$-definable, then $f(a)=b$ just in case $\exists x\forall y\ B(x,y,a,b)$, where $B$ is $\Delta_0$. With an oracle for the halting problem and any particular $x$, $a$ and $b$, we can ask the oracle if the $\forall y$ condition holds. In this way, on input $a$, we can search for an $x$ and $b$ that fulfill the condition. When found, output $b$.

($2\to 1$) If $f$ is computable with respect to an oracle for the halting problem, then it is computable by an inductive Turing machine: just compute better and better approximations to the halting problem, and for each of them, use that approximation as an oracle for the computation of $f$. This process eventually stabilizes, because for any given input, the approximation to the halting problem will be accurate for a long enough time to support the correct computation of $f$. $\Box$

Note that the argument in the implication ($2\to 1$) exhibits the feature that is central to some of the commentary about these machines, namely, that although we can compute better and better approximations to the halting problem, in a way that will eventually be correct on any given instance, nevertheless we are typically not able to recognize computably when our approximation is correct. Thus, although we may be computing the function $f$ accurately by using that approximation, we have no way of knowing for sure that we have the final answer.

Corollary. For any set $A$, the following are equivalent.

  1. $A$ is decidable by an inductive Turing machine.

  2. $A$ is Turing computable from the halting problem.

  3. $A$ has complexity $\Delta_2$ in the arithmetic hierarchy.

Proof. The characteristic function of $A$ is a total function, and so its graph is $\Sigma_2$ if and only if $A$ has complexity $\Delta_2$. $\Box$

In this sense, yes, the so-called inductive Turing machines can compute the halting problem and therefore Hilbert's 10th problem, since that problem is equivalent to the halting problem.

But to be clear, I don't take this to show that the inductive Turing machine model refutes the Church-Turing thesis.

Unfortunately, it seems that much of the commentary and literature surrounding the claim that it does is of poor quality and in some cases mathematically empty. The discussion seems to have become distracted in the literature and gotten off track in a way; it is a pity.

One of the central achievements of computability theory is the recognition of the subtle distinction between the concept of a set being computably enumerable and it being computably decidable. The recognition that these two aspects of computability are not the same has clarified so many issues in computability. We have known since Turing that the halting problem is computably enumerable but not decidable. Meanwhile, the main arguments for inductive computability violating the Church-Turing thesis seem to my way of understanding things to amount to an attempt to erase this important distinction. After all, the halting problem itself is computable in the limit, since we can say that a program does not halt until we see that it does, and then say from that point on that it does halt. Does this show that the halting problem is computably decidable? No, I don't think so, not in any satisfactory way. And similarly I reject that claim that functions computable-in-the-limit are computable. Since these kinds of simple observations seem to resolve essentially all of the issues on this topic, I cannot recommend following much of the literature surrounding this supposed debate.

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  • $\begingroup$ I think the entire claim of inductive turing machine is that they are non-equivalent to TMs with oracles. So from Burgin's view IMs are not same as TMs with Oracles. If you take his superrecursive algorithms book that is what it says and it is a well cited book and this is where my confusion lies. $\endgroup$
    – Turbo
    Commented Jul 29, 2017 at 11:05
  • $\begingroup$ It solves HaltP and Hilb10 without being a TM with oracle (that is the entire hypothesis if you read his comment in blog.computationalcomplexity.org/2007/03/…). So I am still not in clear. $\endgroup$
    – Turbo
    Commented Jul 29, 2017 at 11:06
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    $\begingroup$ Well, I am not saying that inductive Turing machines are the same as Turing machines with an oracle for the halting problem. But I am saying that they compute the same functions. $\endgroup$ Commented Jul 29, 2017 at 11:10
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    $\begingroup$ I don't think anyone has a closed mind about the topic, and certainly the topic of computability-in-the-limit, which as far as I can tell is basically identical to the inductive TM model, is a standard part of the curriculum in computability theory and has been for decades. $\endgroup$ Commented Jul 29, 2017 at 13:39
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    $\begingroup$ For example, the topic of 2-dce, 3-dce, n-dce, $\omega$-dce, which is the study of algorithms that are allowed to change their mind about the output a specific number of times at most, counts now as completely standard classical material. So the standard treatment of computability theory already includes a robust discussion of the functions computable-in-the-limit and far more. So everyone is on board and conversant with the actual mathematics here. What is left out are the contentious, mathematically empty claims that these concepts somehow mean that the Church-Turing thesis is refuted. $\endgroup$ Commented Jul 31, 2017 at 1:02
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Are inductive turing machines physically realizable (at least in the same sense of realizaility of Turing machines as Intel processors with bounded RAM and one that degrades over time)?

Can inductive turing machines solve the halting problem (essentially Hilbert's 10th as well)?

Others have basically answered this question, but if you're still not sure, the following exercise may help you see what's going on.

Forget about inductive Turing machines for the moment. I have just invented a Magic Machine that will solve the halting problem. The Magic Machine is very simple. If you give the Magic Machine a Turing machine T, the Magic Machine will simulate T, and will observe T while the simulation is running. As long as the simulation continues to run, the Magic Machine keeps intoning, "It doesn't halt...it doesn't halt...it doesn't halt..." However, if and when the simulated T halts, the Magic Machine suddenly changes its tune and says, "It halts! It halts! It halts!"

As I said, I claim that my Magic Machine is physically realizable and solves the halting problem. Do you believe me? If so, I will sell you one for the low, low price of $1 million.

If you don't think that my Magic Machine is "a physically realizable machine that solves the halting problem," then see if you can articulate clearly why you aren't inclined to buy it. If you can, then you should be able to articulate equally clearly why no inductive Turing machine is "a physically realizable machine that solves the halting problem."

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  • $\begingroup$ Thanks, Timothy, this is very nice. I was trying to convey the same example in the last paragraph of my answer, but you did so much more colorfully! $\endgroup$ Commented Jul 30, 2017 at 22:55
  • $\begingroup$ @timothychow is this also what you have in mind 'A simple inductive Turing machine produces its results without stopping. It is possible that in the sequence of computations after some step, the word on the output tape is not changing, while the simple inductive Turing machine continues working. This word, which is not changing is the result Thus the which is not changing, is the result. Thus, the simple inductive simple inductive Turing machine does not halt...'? idt.mdh.se/kurser/cd5560/12_11/LECTURES/pdf/…. $\endgroup$
    – Turbo
    Commented Jul 30, 2017 at 23:18
  • $\begingroup$ @timothychow in his book he says 'Although many still believe that only recursive algorithms exist and that only some of them are realizable, there are many situations in which people actually work with superrecursive algorithms' and continues examples of superrecursive algorithms include ITMs (books.google.com/…). $\endgroup$
    – Turbo
    Commented Jul 30, 2017 at 23:24
  • $\begingroup$ @timothychow here he might inductive TMs and limit TMs pdfs.semanticscholar.org/fb01/… (I am not sure though). 'Such big networks as INTERNET give another important example of a situation in which conventional algorithms are not adequate. Network functioning is organized by algorithms embodied in a multiplicity of different programs. It is generally assumed that any computer program, is a conventional, i.e., recursive algorithm.. $\endgroup$
    – Turbo
    Commented Jul 30, 2017 at 23:28
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    $\begingroup$ @777 : Instead of constantly quoting other people, I recommend that you answer the questions yourself. Specifically, do you believe that my Magic Machine is "a physically realizable machine that solves the halting problem"? If not, why not? If anything is physically realizable, surely the Magic Machine is. And it seems to do as good a job as any inductive Turing machine at "solving the halting problem." Doesn't it? Please answer for yourself instead of quoting someone else. $\endgroup$ Commented Jul 31, 2017 at 1:00
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This is not an answer to the OP's question, and is a bit of a tangent. But perhaps relevant concerning the physical realizability issue raised by Joel.

I just today heard a talk on a "Fold-and-Cut Machine." This leads to a physical model equi-powerful to a nondeterministic Turing machine:

"a fold-and-cut machine can decide a 3-SAT instance with $n$ variables and $m$ clauses using $O(nm+m^2)$ operations (...), showing that the machine is at least as powerful as a nondeterministic Turing machine."

An, Byoungkwon, Erik D. Demaine, Martin L. Demaine, and Jason S. Ku. "Computing 3SAT on a Fold-and-Cut Machine." Full CCCG Proceedings download, p.208ff for the article.

One folds a strip of paper a polynomial number of times, snips it to produce holes, rearranges the folding, and then looks to see if you can see all the way through the rearranged folding. You can iff the 3-SAT instance is solvable.


SATholes


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Consider the following paper, written by A. Steven Younger, Emmett Redd, Hava Siegelmann, and Conrad Bell:

"A Physical Machine Based on a Super-Turing Computational Model" [found under title on the Web].

I quote the Abstract verbatim:

We present evidence that the Turing Machine is too restrictive a model to sufficiently describe the computation of our analog computer and, therefore a more conprehensive model is needed. We report on the construction of a prototype, the Optical Analog Recurrent Neural Network (OpticARRN), and experimental results showing that it performs computations which are beyond those of computers based on the Turing machine. we conclude that the behavior of OpticARRN is better described by the super-Turing computational model proposed by Siegelmann. To the best of our knowledge, this is the first application of analog recurrent neural networks realized in a physical computer based on this model.

(Suffice it to say, I leave the judgement as to the truth or falsity of the claim(s) made in the Abstract and the paper to the Reader.)

What I find personally (for what that's worth) interesting in this paper is this particular claim (found in Section 3, "Testing for Computation Beyond the Turing Limit"):

In order to test super-Turing computation, a suitable problem must be found. In this case, the answer came from the area of chaotic systems. The dynamics of chaos are both aperiodic and defined on a continuous phase space. As such, they cannot be mimicked by a Turing machine [Siegelmann, H. (1998). Neural Networks and Analog Computation Beyond the Turing Limit. Boston: Birkhauser(p. 155)].

It is the validity of this claim that (seemingly) makes or breaks the experiment. Also, the reader should pay particular attention to their methodology for the interpretation of the experimental data.

Note that the experimental setup is shown in figure 1 on pg. 4 of the paper (at least on the copy of the paper I found on the Web).

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  • $\begingroup$ Why the downvote? $\endgroup$ Commented Apr 13, 2018 at 14:19

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