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I ask the question because of the following statement found in Mark Burgin's paper, "Algorithmic complexity of recursive and inductive algorithms", Theoretical Computer Science 317 (2004) 31-60 (pg. 34):

It is usually assumed that any finite set is recursively computable and even decidable. When a finite set is given by a list, then this is true. However, this assumption is not valid in a general case when a finite set can be defined by a description [the 'presentation'--my comment]. For instance, let us take the set $X$ of all indices of those Turing machines that have the length of their description less than 1000 and that do not terminate on some input with the length less than 1000. This set is finite, but it is not recursively computable (enumerable).

The problem I see with Prof. Burgin's statement is that computability theory can be (and is usually) formulated in $ZFC$ (the background set theory) in which all sets (regardless of description) can be well-ordered (it should be noted that well-ordered sets, finite or otherwise, can be identified as lists--see Oliver Deiser's paper, "An Axiomatic Theory of Well-Orderings", The Review of Symbolic Logic Volume 4, Issue 2, June 2011, pp. 186-204). As regards finite sets, the Diaconescu-Goodman-Myhill theorem (taken from nlab)

The following are equivalent:

  1. The principle of excluded middle

  2. Finitely indexed sets are projective (in fact, it suffices 2-indexed sets to be projective)

  3. Finite sets are choice (in fact, it suffices for $2$ to be choice) (Here, a set $A$ is finite or finitely-indexed (respectively) if, for some natural number n, there is a bijection or surjection (respectively) {0,...,n $-$ 1} $\rightarrow $$A$. Also, $A$ is projective iff every entire relation from $A$ to $B$ (so that every element of $A$ is related to some element of $B$), for any $B$, contains a function $A$ $\rightarrow$ $B$, while $B$ is choice iff every entire relation from $A$ to $B$, for any $A$, contains a function $A$ $\rightarrow$ $B$.)

suggests that Prof. Burgin's example should lead one to question the principle of excluded middle, much as the following:

The 2$\uparrow$$\uparrow$$\uparrow$6'th digit in the decimal expansion of $\pi$ = 1

might.

Does it (in the sense that Brouwer's argument against excluded middle would cause one to question it)?

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Burgin seems to conflate the notion of computability (existence of an algorithm) with a stronger notion such as our knowing an algorithm or existence of a proof that a particular algorithm agrees with the given presentation.

In his example, the finite set $X$ is computable. Indeed, any finite set is computable by a table look-up algorithm. For Burgin's $X$, we don't know the table, so we don't know the algorithm, and we have no effective way to produce the table from the given description of $X$. But that doesn't make $X$ uncomputable; it just says that $X$ doesn't have the stronger properties mentioned above.

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  • $\begingroup$ Thank you for the very nice answer--it is very helpful. As regards the table look-up algorithm--would it well-order $A$ making it a list? $\endgroup$ Commented Nov 7, 2021 at 3:21
  • $\begingroup$ Rather, would the table (if one had it) well-order $A$ making it a list? $\endgroup$ Commented Nov 7, 2021 at 3:33
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    $\begingroup$ @ThomasBenjamin Our means of expressing a table (or any other algorithm for that matter) involve writing the steps in some linear order, so they would implicitly involve a linear ordering of the set. Note, though, that the usual notion of a finite set --- existence of a bijection with a proper initial segment of the natural numbers --- also implicitly involves a well-ordering. $\endgroup$ Commented Nov 7, 2021 at 13:30
  • $\begingroup$ I am wondering, though, if having an effective way to produce the table from the given description of $X$ (in Burgin's example or in any other example, for that matter) is implicit in our understanding $\endgroup$ Commented Nov 8, 2021 at 3:47
  • $\begingroup$ (cont.) of what it means to 'have an algorithm' for generating a given finite set (Burgin grants that for lists, i.e., well-ordered finite sets, what you say is true --see my quote of his in my question). $\endgroup$ Commented Nov 8, 2021 at 4:01

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