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Computation in the limit

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In computability theory, a function is called limit computable if it is the limit of a uniformly computable sequence of functions. The terms computable in the limit, limit recursive and recursively approximable are also used. One can think of limit computable functions as those admitting an eventually correct computable guessing procedure at their true value. A set is limit computable just when its characteristic function is limit computable.

If the sequence is uniformly computable relative to D, then the function is limit computable in D.

Contents

Formal definitionEdit

A total function   is limit computable if there is a total computable function   such that

 

The total function   is limit computable in D if there is a total function   computable in D also satisfying

 

A set of natural numbers is defined to be computable in the limit if and only if its characteristic function is computable in the limit. In contrast, the set is computable if and only if it is computable in the limit by a function   and there is a second computable function that takes input i and returns a value of t large enough that the   has stabilized.

Limit lemmaEdit

The limit lemma states that a set of natural numbers is limit computable if and only if the set is computable from   (the Turing jump of the empty set). The relativized limit lemma states that a set is limit computable in   if and only if it is computable from  . Moreover, the limit lemma (and its relativization) hold uniformly. Thus one can go from an index for the function   to an index for   relative to  . One can also go from an index for   relative to   to an index for some   that has limit  .

ProofEdit

As   is a [computably enumerable] set, it must be computable in the limit itself as the computable function can be defined

 

whose limit   as   goes to infinity is the characteristic function of  .

It therefore suffices to show that if limit computability is preserved by Turing reduction, as this will show that all sets computable from   are limit computable. Fix sets   which are identified with their characteristic functions and a computable function   with limit  . Suppose that   for some Turing reduction   and define a computable function   as follows

 

Now suppose that the computation   converges in   steps and only looks at the first   bits of  . Now pick   such that for all    . If   then the computation   converges in at most   steps to  . Hence   has a limit of  , so   is limit computable.

As the   sets are just the sets computable from   by Post's theorem, the limit lemma also entails that the limit computable sets are the   sets.

Limit computable real numbersEdit

A real number x is computable in the limit if there is a computable sequence   of rational numbers (or, which is equivalent, computable real numbers) which converges to x. In contrast, a real number is computable if and only if there is a sequence of rational numbers which converges to it and which has a computable modulus of convergence.

When a real number is viewed as a sequence of bits, the following equivalent definition holds. An infinite sequence   of binary digits is computable in the limit if and only if there is a total computable function   taking values in the set   such that for each i the limit   exists and equals  . Thus for each i, as t increases the value of   eventually becomes constant and equals  . As with the case of computable real numbers, it is not possible to effectively move between the two representations of limit computable reals.

ExamplesEdit

  • The real whose binary expansion encodes the halting problem is computable in the limit but not computable.
  • The real whose binary expansion encodes the truth set of first-order arithmetic is not computable in the limit.

See alsoEdit

ReferencesEdit

  1. J. Schmidhuber, "Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit", International Journal of Foundations of Computer Science, 2002.
  2. R. Soare. Recursively Enumerable Sets and Degrees. Springer-Verlag 1987.