I am trying to show that a decision problem is in $\Pi_1^1$. Because of this, I am looking for:

  1. Papers or books that present a complete and well-explained proof where a problem is shown to be in $\Pi_1^1$.
  2. Well-known problems that are in $\Pi_1^1$.

Any suggestions?


2 Answers 2


I think that a good well-studied problem may be the "recurrent tiling problem". For more references (with many use cases) consult the paper "Recurring Dominoes: Making the Highly Undecidable Highly Understandable" by David Harel [Sec. 2.1, 1].

[1] https://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/RucurringDominoes.pdf

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    $\begingroup$ A quite readable paper on tilings is also “The convenience of tilings” by Peter van Emde Boas $\endgroup$ Commented Oct 20, 2023 at 20:16
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    $\begingroup$ Thanks guys for the comments! At the end we were able to use some of the problems presented by Harel to reduce to and from : ) $\endgroup$ Commented Feb 8 at 11:08

The standard example of a $\Pi^1_1$ complete decision problem is the following:

Is $e$ an index for a Turing machine which (halts on all inputs and) computes a well-ordering of $\mathbb{N}$?

Let's write "$\mathcal{W}$" for the set of such $e$s.

Here, a well-ordering of $\mathbb{N}$ is a binary relation on $\mathbb{N}$ which is a linear order with the additional property that every (nonempty) subset has a least element. One inessential (but technically useful) variant of this is Kleene's $\mathcal{O}$, which basically focuses on "nicely presented" relations. Re: my parenthetical, detecting whether a Turing machine halts on all inputs is merely $\Pi^0_2$, so galactically weaker than what we're interested in.

It's a standard result (and a fun exercise to show) that $\mathcal{W}$ is $\Pi^1_1$ complete; that is, a decision problem is $\Pi^1_1$ iff it can be (computably many-one) reduced to $\mathcal{W}$. So $\mathcal{W}$ serves as a template for testing $\Pi^1_1$-ness analogously to the role that the halting problem serves for $\Sigma^0_1$-ness or (replacing "computably" with "polynomial-time-computably") that $\mathsf{SAT}$ serves for $\mathsf{NP}$-ness.

The basic idea behind proving that $\mathcal{W}$ (or $\mathcal{O}$) is $\Pi^1_1$ hard (this is the nontrivial part of $\Pi^1_1$ completeness in these cases) is as follows: given an arbitrary $\Sigma^1_1$ definition $\varphi$ and a number $n$, we build a tree $T_{n,\varphi}$ whose nodes represent "apparent counterexamples" to $\varphi(n)$ holding. An infinite path through $T_{n,\varphi}$ corresponds to a genuine counterexample to $\varphi(n)$, so we've reduced checking whether $\varphi(n)$ holds to checking whether $T_{n,\varphi}$ is well-founded. This isn't quite the same as "well-ordered," but now we use the Kleene-Brouwer construction to reduce well-foundedness to well-orderedness.

A nice general treatment of $\Pi^1_1$ complete sets and their properties - but focusing on the more technical $\mathcal{O}$ rather than $\mathcal{W}$ - can be found in the first section of Sacks' book Higher recursion theory.

EDIT: This next bit is really more set theory than complexity theory, but I think it's still worth mentioning:

One key bit of intuition (due to Kreisel if I have my history right) is that despite appearances $\Pi_1^1$ is the analogue of computably enumerable (c.e.) rather than co-c.e. Even more confusingly, $\Delta^1_1$ is the analogue of finite rather than computable. (For an example of the latter point, the $\Pi^1_1$ image of a $\Delta^1_1$ set is $\Delta^1_1$ again.)

This is explained via $\omega_1^{CK}$-recursion theory - the special case of $\alpha$-recursion theory with $\alpha=\omega_1^{CK}$ - as follows (with "$L_\alpha$" denoting the $\alpha$th level of Godel's constructible universe):

  • Classically, shifting from the natural numbers to the hereditarily finite sets we have "finite" = "element of $L_\omega$," and "c.e." = "$\Sigma_1(L_\omega)$."

  • The generalization to $\omega_1^{CK}$ is then "$\omega_1^{CK}$-finite" = "element of $L_{\omega_1^{CK}}$" and "$\omega_1^{CK}$-c.e." = "$\Sigma_1(L_{\omega_1^{CK}})$."

Note that $\omega$ (or $\mathbb{N}$ if you prefer) is $\omega_1^{CK}$-finite. It's not hard to show that $X\subseteq\omega$ is $\Pi^1_1$ in the usual sense iff it is $\Sigma_1(L_{\omega_1^{CK}})$. Conversely, though, we lose a crucial bit of combinatorics in passing to $\omega_1^{CK}$: a c.e. subset of a finite set is not finite but merely "subfinite." However, a computable subset of a finite set is finite. This corresponds to the fact that the theory $\mathsf{KP}$ (= basically the amount of set theory that $L_{\omega_1^{CK}}$ satisfies) entails $\Delta_1$ comprehension but not $\Sigma_1$ comprehension.

Why on earth am I saying all this? Well, in my opinion a lot of the properties of $\Pi^1_1$ sets become much simpler when thought of in terms of $\omega_1^{CK}$-recursion theory. If set theory puts you off, you may find "metarecursion theory" - basically $\omega_1^{CK}$-recursion theory but over (paths through) Kleene's $\mathcal{O}$ instead of $L_{\omega_1^{CK}}$ as such - to be a useful rephrasing; again assuming my history is right, this was how things were originally phrased by Kreisel and Sacks before the simplifying power of the shift to $\alpha$-recursion theory was fully understood.

  • $\begingroup$ Out of curiosity, why the downvote? $\endgroup$ Commented Oct 30, 2023 at 19:11
  • $\begingroup$ Thank you for your detailed reply! : ) $\endgroup$ Commented Feb 8 at 11:09

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