arXiv subjects

Denis R. Hirschfeldt

Publications and source records attributed to Denis R. Hirschfeldt.

14 recordsLinked to original sources

arXiv2026

The Complexity of the Set of Validities of a Theory

We study the collection of first-order logical schemata all of whose instances are theorems of a given theory $T$; we call these the validities of $T$ ($\mathsf{V}(T)$). It is easy to see that if $T$ is a decidable theory, then $\mathsf{V}(T)$ is distinct from the set of valid formulas of first-order logic as customarily understood. We provide a complete model-theoretic characterization of the complexity, in the sense of Turing degree, of $\mathsf{V}(T)$ for decidable theories $T$, and answer a question posed by Vaught in 1960 concerning the complexity of the collection of validities common to all decidable theories.

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arXiv2022

Thin Set Versions of Hindman's Theorem

In this paper we examine the reverse mathematical strength of a variation of Hindman's Theorem HT constructed by essentially combining HT with the Thin Set Theorem TS to obtain a principle which we call thin-HT. thin-HT says that every coloring $c: \mathbb{N} \to \mathbb{N}$ has an infinite set $S \subseteq \mathbb{N}$ whose finite sums are thin for $c$, meaning that there is an $i$ with $c(s) \neq i$ for all $s \in S$. We show that there is a computable instance of thin-HT such that every solution computes $\emptyset'$, as is the case with HT (see Blass, Hirst, and Simpson 1987). In analyzing this proof, we deduce that thin-HT implies $ACA_0$ over $RCA_0 + IΣ^0_2$. On the other hand, using Rumyantsev and Shen's computable version of the Lovász Local Lemma, we show that there is a computable instance of the restriction of thin-HT to sums of exactly 2 elements such that any solution has diagonally noncomputable degree relative to $\emptyset'$. Hence there is a computable instance of this restriction of thin-HT with no $Σ^0_2$ solution.

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arXiv2021

Reduction games, provability, and compactness

Hirschfeldt and Jockusch (2016) introduced a two-player game in which winning strategies for one or the other player precisely correspond to implications and non-implications between $Π^1_2$ principles over $ω$-models of $\mathsf{RCA}_0$. They also introduced a version of this game that similarly captures provability over $\mathsf{RCA}_0$. We generalize and extend this game-theoretic framework to other formal systems, and establish a certain compactness result that shows that if an implication $\mathsf{Q} \to \mathsf{P}$ between two principles holds, then there exists a winning strategy that achieves victory in a number of moves bounded by a number independent of the specific run of the game. This compactness result generalizes an old proof-theoretic fact noted by H.~Wang (1981), and has applications to the reverse mathematics of combinatorial principles. We also demonstrate how this framework leads to a new kind of analysis of the logical strength of mathematical problems that refines both that of reverse mathematics and that of computability-theoretic notions such as Weihrauch reducibility, allowing for a kind of fine-structural comparison between $Π^1_2$ principles that has both computability-theoretic and proof-theoretic aspects, and can help us distinguish between these, for example by showing that a certain use of a principle in a proof is "purely proof-theoretic", as opposed to relying on its computability-theoretic strength. We give examples of this analysis to a number of principles at the level of $\mathsf{B}Σ^0_2$, uncovering new differences between their logical strengths.

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arXiv2021

A Feiner Look at the Intermediate Degrees

We say that a set $S$ is $Δ^0_{(n)}(X)$ if membership of $n$ in $S$ is a $Δ^0_{n}(X)$ question, uniformly in $n$. A set $X$ is low for $Δ$-Feiner if every set $S$ that is $Δ^0_{(n)}(X)$ is also $Δ^0_{(n)}(\emptyset)$. It is easy to see that every low$_n$ set is low for $Δ$-Feiner, but we show that the converse is not true by constructing an intermediate c.e. set that is low for $Δ$-Feiner. We also study variations on this notion, such as the sets that are $Δ^0_{(bn+a)}(X)$, $Σ^0_{(bn+a)}(X)$, or $Π^0_{(bn+a)}(X)$, and the sets that are low, intermediate, and high for these classes. In doing so, we obtain a result on the computability of Boolean algebras, namely that there is a Boolean algebra of intermediate c.e. degree with no computable copy.

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arXiv2021

Coarse computability, the density metric, Hausdorff distances between Turing degrees, perfect trees, and reverse mathematics

The coarse similarity class $[A]$ of $A$ is the set of all $B$ whose symmetric difference with $A$ has asymptotic density 0. There is a natural metric $δ$ on the space $\mathcal{S}$ of coarse similarity classes defined by letting $δ([A],[B])$ be the upper density of the symmetric difference of $A$ and $B$. We study the resulting metric space, showing in particular that between any two distinct points there are continuum many geodesic paths. We also study subspaces of the form $\{[A] : A \in \mathcal U\}$ where $\mathcal U$ is closed under Turing equivalence, and show that there is a tight connection between topological properties of such a space and computability-theoretic properties of $\mathcal U$. We then define a distance between Turing degrees based on Hausdorff distance in this metric space. We adapt a proof of Monin to show that the distances between degrees that occur are exactly 0, 1/2, and 1, and study which of these values occur most frequently in the senses of measure and category. We define a degree to be attractive if the class of all degrees at distance 1/2 from it has measure 1, and dispersive otherwise. We study the distribution of attractive and dispersive degrees. We also study some properties of the metric space of Turing degrees under this Hausdorff distance, in particular the question of which countable metric spaces are isometrically embeddable in it, giving a graph-theoretic sufficient condition. We also study the computability-theoretic and reverse-mathematical aspects of a Ramsey-theoretic theorem due to Mycielski, which in particular implies that there is a perfect set whose elements are mutually 1-random, as well as a perfect set whose elements are mutually 1-generic. Finally, we study the completeness of $(\mathcal S,δ)$ from the perspectives of computability theory and reverse mathematics.

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arXiv2020

Some results concerning the $\mathsf{SRT}^2_2$ vs. $\mathsf{COH}$ problem

The $\mathsf{SRT}^2_2$ vs.\ $\mathsf{COH}$ problem is a central problem in computable combinatorics and reverse mathematics, asking whether every Turing ideal that satisfies the principle $\mathsf{SRT}^2_2$ also satisfies the principle $\mathsf{COH}$. This paper is a contribution towards further developing some of the main techniques involved in attacking this problem. We study several principles related to each of $\mathsf{SRT}^2_2$ and $\mathsf{COH}$, and prove results that highlight the limits of our current understanding, but also point to new directions ripe for further exploration.

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arXiv2019

A minimal pair in the generic degrees

We show that there is a minimal pair in the nonuniform generic degrees, and hence also in the uniform generic degrees. This fact contrasts with Igusa's result that there are no minimal pairs for relative generic computability, and answers a basic structural question mentioned in several papers in the area.

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arXiv2019

Ramsey's theorem and products in the Weihrauch degrees

We study the positions in the Weihrauch lattice of parallel products of various combinatorial principles related to Ramsey's theorem. Among other results, we obtain an answer to a question of Brattka, by showing that Ramsey's theorem for pairs ($\mathsf{RT}^2_2$) is strictly Weihrauch below the parallel product of the stable Ramsey's theorem for pairs and the cohesive principle ($\mathsf{SRT}^2_2 \times \mathsf{COH}$).

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arXiv2018

Combinatorial principles equivalent to weak induction

We consider two combinatorial principles, ${\sf{ERT}}$ and ${\sf{ECT}}$. Both are easily proved in ${\sf{RCA}}_0$ plus ${Σ^0_2}$ induction. We give two proofs of ${\sf{ERT}}$ in ${\sf{RCA}}_0$, using different methods to eliminate the use of ${Σ^0_2}$ induction. Working in the weakened base system ${\sf{RCA}}_0^*$, we prove that ${\sf{ERT}}$ is equivalent to ${Σ^0_1}$ induction and ${\sf{ECT}}$ is equivalent to ${Σ^0_2}$ induction. We conclude with a Weihrauch analysis of the principles, showing ${\sf{ERT}} {\equiv_{\rm W}} {\sf{LPO}}^* {<_{\rm W}}{\sf{TC}_{\mathbb N}}^* {\equiv_{\rm W}} {\sf{ECT}}$.

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arXiv2018

Dense computability, upper cones, and minimal pairs

This paper concerns algorithms that give correct answers with (asymptotic) density $1$. A dense description of a function $g : ω\to ω$ is a partial function $f$ on $ω$ such that $\left\{n : f(n) = g(n)\right\}$ has density $1$. We define $g$ to be densely computable if it has a partial computable dense description $f$. Several previous authors have studied the stronger notions of generic computability and coarse computability, which correspond respectively to requiring in addition that $g$ and $f$ agree on the domain of $f$, and to requiring that $f$ be total. Strengthening these two notions, call a function $g$ effectively densely computable if it has a partial computable dense description $f$ such that the domain of $f$ is a computable set and $f$ and $g$ agree on the domain of $f$. We compare these notions as well as asymptotic approximations to them that require for each $ε> 0$ the existence of an appropriate description that is correct on a set of lower density of at least $1 - ε$. We determine which implications hold among these various notions of approximate computability and show that any Boolean combination of these notions is satisfied by a c.e. set unless it is ruled out by these implications. We define reducibilities corresponding to dense and effectively dense reducibility and show that their uniform and nonuniform versions are different. We show that there are natural embeddings of the Turing degrees into the corresponding degree structures, and that these embeddings are not surjective and indeed that sufficiently random sets have quasiminimal degree. We show that nontrivial upper cones in the generic, dense, and effective dense degrees are of measure $0$ and use this fact to show that there are minimal pairs in the dense degrees.

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arXiv2018

The reverse mathematics of Hindman's theorem for sums of exactly two elements

Hindman's Theorem (HT) states that for every coloring of $\mathbb N$ with finitely many colors, there is an infinite set $H \subseteq \mathbb N$ such that all nonempty sums of distinct elements of $H$ have the same color. The investigation of restricted versions of HT from the computability-theoretic and reverse-mathematical perspectives has been a productive line of research recently. In particular, HT$^{\leqslant n}_k$ is the restriction of HT to sums of at most $n$ many elements, with at most $k$ colors allowed, and HT$^{=n}_k$ is the restriction of HT to sums of \emph{exactly} $n$ many elements and $k$ colors. Even HT$^{\leqslant 2}_2$ appears to be a strong principle, and may even imply HT itself over RCA$_0$. In contrast, HT$^{=2}_2$ is known to be strictly weaker than HT over RCA$_0$, since HT$^{=2}_2$ follows immediately from Ramsey's Theorem for $2$-colorings of pairs. In fact, it was open for several years whether HT$^{=2}_2$ is computably true. We show that HT$^{=2}_2$ and similar results with addition replaced by subtraction and other operations are not provable in RCA$_0$, or even WKL$_0$. In fact, we show that there is a computable instance of HT$^{=2}_2$ such that all solutions can compute a function that is diagonally noncomputable relative to $\emptyset'$. It follows that there is a computable instance of HT$^{=2}_2$ with no $Σ^0_2$ solution, which is the best possible result with respect to the arithmetical hierarchy. Furthermore, a careful analysis of the proof of the result above about solutions DNC relative to $\emptyset'$ shows that HT$^{=2}_2$ implies RRT$^{=2}_2$, the Rainbow Ramsey Theorem for $2$-colorings of pairs, over RCA$_0$. The most interesting aspect of our construction of computable colorings as above is the use of an effective version of the Lovász Local Lemma due to Rumyantsev and Shen.

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arXiv2015

Asymptotic density and the coarse computability bound

For $r \in [0,1]$ we say that a set $A \subseteq ω$ is \emph{coarsely computable at density} $r$ if there is a computable set $C$ such that $\{n : C(n) = A(n)\}$ has lower density at least $r$. Let $γ(A) = \sup \{r : A \hbox{ is coarsely computable at density } r\}$. We study the interactions of these concepts with Turing reducibility. For example, we show that if $r \in (0,1]$ there are sets $A_0, A_1$ such that $γ(A_0) = γ(A_1) = r$ where $A_0$ is coarsely computable at density $r$ while $A_1$ is not coarsely computable at density $r$. We show that a real $r \in [0,1]$ is equal to $γ(A)$ for some c.e.\ set $A$ if and only if $r$ is left-$Σ^0_3$. A surprising result is that if $G$ is a $Δ^0_2$ $1$-generic set, and $A \leq\sub{T} G$ with $γ(A) = 1$, then $A$ is coarsely computable at density $1$.

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arXiv2015

Coarse Reducibility and Algorithmic Randomness

A coarse description of a subset A of omega is a subset D of omega such that the symmetric difference of A and D has asymptotic density 0. We study the extent to which noncomputable information can be effectively recovered from all coarse descriptions of a given set A, especially when A is effectively random in some sense. We show that if A is 1-random and B is computable from every coarse description D of A, then B is K-trivial, which implies that if A is in fact weakly 2-random then B is computable. Our main tool is a kind of compactness theorem for cone-avoiding descriptions, which also allows us to prove the same result for 1-genericity in place of weak 2-randomness. In the other direction, we show that if A is a 1-random set which Turing-reduces to 0', then there is a noncomputable c.e. set computable from every coarse description of A, but that not all K-trivial sets are computable from every coarse description of some 1-random set. We study both uniform and nonuniform notions of coarse reducibility. A set Y is uniformly coarsely reducible to X if there is a Turing functional Phi such that if D is a coarse description of X, then Phi^D is a coarse description of Y. A set B is nonuniformly coarsely reducible to A if every coarse description of A computes a coarse description of B. We show that a certain natural embedding of the Turing degrees into the coarse degrees (both uniform and nonuniform) is not surjective. We also show that if two sets are mutually weakly 3-random, then their coarse degrees form a minimal pair, in both the uniform and nonuniform cases, but that the same is not true of every pair of relatively 2-random sets, at least in the nonuniform coarse degrees.

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arXiv2014

The Strength of Some Combinatorial Principles Related to Ramsey's Theorem for Pairs

We study the reverse mathematics and computability-the\-o\-re\-tic strength of (stable) Ramsey's Theorem for pairs and the related principles COH and DNR. We show that SRT$^2_2$ implies DNR over RCA$_0$ but COH does not, and answer a question of Mileti by showing that every computable stable $2$-coloring of pairs has an incomplete $Δ^0_2$ infinite homogeneous set. We also give some extensions of the latter result, and relate it to potential approaches to showing that SRT$^2_2$ does not imply RT$^2_2$.

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