Publications and preprints
Artificial Intelligence
(with Jacopo Amidei, Duccio Pianigiani, Luca San Mauro, Andrea Sorbi )
Journal of Logic and Computation, 29 (2019), no. 1, 157-184.
Abstract:
This paper is part of a project that is based on the notion of a dialectical system, introduced by Magari as a way of capturing trial and error mathematics. In Amidei et al. (2016, Rev. Symb. Logic, 9, 1–26) and Amidei et al. (2016, Rev. Symb. Logic, 9, 299–324), we investigated the expressive and computational power of dialectical systems, and we compared them to a new class of systems, that of quasi-dialectical systems, that enrich Magari’s systems with a natural mechanism of revision. In the present paper we consider a third class of systems, that of
(with Luca San Mauro )
in 20th Conference in Theoretical Aspects of Rationality and Knowledge (TARK 2025), 2025
Abstract:
Argumentation frameworks, consisting of arguments and an attack relation representing conflicts, are fundamental for formally studying reasoning under conflicting information. We use methods from mathematical logic, specifically computability and set theory, to analyze the grounded extension, a widely-used model of maximally skeptical reasoning, defined as the least fixed-point of a natural defense operator. Without additional constraints, finding this fixed-point requires transfinite iterations. We identify the exact ordinal number corresponding to the length of this iterative process and determine the complexity of deciding grounded acceptance, showing it to be maximally complex. This shows a marked distinction from the finite case where the grounded extension is polynomial-time computable, thus simpler than other reasoning problems explored in formal argumentation.
(with Luca San Mauro )
in 19th edition of the European Conference on Logics in Artificial Intelligence (JELIA 2025), 2025
Abstract:
Dialectical systems are a mathematical formalism for modeling an agent updating a knowledge base seeking consistency. Introduced in the 1970s by Roberto Magari, they were originally conceived to capture how a working mathematician or a research community refines beliefs in the pursuit of truth. Dialectical systems also serve as natural models for the belief change of an automated agent, offering a unifying, computable framework for dynamic belief management.
The literature distinguishes three main models of dialectical systems: (d-)dialectical systems based on revising beliefs when they are seen to be inconsistent, p-dialectical systems based on revising beliefs based on finding a counterexample, and q-dialectical systems which can do both.
We answer an open problem in the literature by proving that q-dialectical systems are strictly more powerful than p-dialectical systems, which are themselves known to be strictly stronger than (d-)dialectical systems. This result highlights the complementary roles of counterexample and contradiction in automated belief revision, and thus also in the reasoning processes of mathematicians and research communities.
(with Luca San Mauro )
in 19th edition of the European Conference on Logics in Artificial Intelligence (JELIA 2025), 2025
Abstract:
Argumentation frameworks (AFs) are a foundational tool in artificial intelligence for modeling structured reasoning and conflict. SCC-recursiveness is a well-known design principle in which the evaluation of arguments is decomposed according to the strongly connected components (SCCs) of the attack graph, proceeding recursively from "higher" to "lower" components. While SCC-recursive semantics such as cf2 and stg2 have proven effective for finite AFs, Baumann and Spanring showed the failure of SCC-recursive semantics to generalize reliably to infinite AFs due to issues with well-foundedness.
We propose two approaches to extending SCC-recursiveness to the infinite setting. We systematically evaluate these semantics using Baroni and Giacomin’s established criteria, showing in particular that directionality fails in general. We then examine these semantics' behavior in finitary frameworks, where we find some of our semantics satisfy directionality. These results advance the theory of infinite argumentation and lay the groundwork for reasoning systems capable of handling unbounded or evolving domains.
Recursive Model Theory
Thesis
Abstract:
We employ the Hrushovski Amalgamation Construction to generate strongly minimal examples of interesting recursive model theoretic phenomena. We show that there exists a strongly minimal theory whose only recursively presentable models are prime or saturated. We show that there exists a strongly minimal theory in a language with finite signature whose only recursively presentable model is the saturated model.Similarly, we show that for every
Annals of Pure and Applied Logic, 162 (2011), 367-372
Abstract:
We describe strongly minimal theories
Annals of Pure and Applied Logic, 162 (2011), 367-372
Abstract:
We describe strongly minimal theories
Proceedings of the AMS, 141 (2013), 2501-2514
Abstract:
We show that even for categorical theories, recursiveness of the models guarantees no information regarding the complexity of the theory. In particular, we show that every tt-degree reducible to
(with Tamvana Makuluni )
Israel Journal of Mathematics, 196 (2013), 491--498
Abstract:
We classify the complexity of the index set of uncountably categorical theories. We show that this index set surprisingly falls at the intermediate stage of being complete for intersections of
(with Julia F. Knight )
Journal of Symbolic Logic, 78 (2013), 1189--1198
Abstract:
For a countable structure
(with Alice Medvedev )
Transactions of The AMS, 366 (2014), 2393--2417
Abstract:
We conjecture that for a strongly minimal theory
(with Asher Kach )
Algebra and Logic, 53 (2014), no. 2, 176--183
Abstract:
We study, for a countably categorical theory
Lobachevskii Journal of Mathematics, 2014, Volume 35, Issue 4, 287-291
Abstract:
This paper gives an overview of results about the spectra of computable (or recursive) models of strongly minimal disintegrated theories. Spectra of strongly minimal disintegrated theories in finite languages and in languages consisting of binary relation symbols will be described.
Journal of Symbolic Logic, 79 (2014), 186--192
Abstract:
We characterize
(with Joseph S. Miller )
Proceeding of the AMS, 143 (2015), 1283--1298
Abstract:
We introduce the notion of a degree spectrum of a theory to be the set of Turing degrees which compute some model of the theory. We generate examples showing that not all degree spectra of theories are degree spectra of structures and vice-versa. To this end, we give a new necessary condition on the degree spectrum of a structure, specifically showing that the set of PA degrees is not a degree spectrum of a structure but is a degree spectrum of a theory.
(with Dmitriy Dushenin, Cameron Hill, Julia F. Knight, Alexander Melnikov )
Algebra and Logic, Vol. 54, No. 6, January, 2016, 489--501
Abstract:
The notion of Turing computable embedding provides a computable analog of Borel embedding, and a way to effectively compare classes of countable structures, reducing the classification problem for one class to that for the other. Most of the known results on non-existence of Turing computable embeddings reflect differences in the complexity of the sentences needed to distinguish among non-isomorphic members of the two classes. Here we consider sum structures. We show that the
(with Mingzhong Cai, Iskander Sh. Kalimullin, Steffen Lempp, Joseph S. Miller, Antonio Montalban )
Journal of Symbolic Logic, 81 (2016), 997–1006
Abstract:
We study Turing degrees
(with Mingzhong Cai, David Diamondstone, Steffen Lempp, Joseph S. Miller )
Transactions of The AMS, 369 (2017), 6493-6510
Abstract:
We analyze the spectra of theories which are
(with Steffen Lempp )
Submitted
Abstract:
In this paper, we consider the spectra of computable (or recursive) models of strongly minimal disintegrated theories. We conjecture that for any
(with Julia F. Knight )
Journal of the European Mathematical Society, 20 (2018), 1561-1594
Abstract:
We give effectiveness conditions on a strongly minimal theory
(with Steffen Lempp, Noah D. Schweber )
Advanced in Mathematics, Volume 386, 6 August 2021
Abstract:
What information does one need to know in order to build the models of a strongly minimal theory? To answer this question, we first formalize it in two ways. Note that if a theory
(with Julia F. Knight, Rutger Kuyper, Joseph S. Miller, Mariya I. Soskova )
The Journal of Symbolic Logic , Volume 88 , Issue 3 , September 2023 , pp. 1083 - 1102
Abstract:
We study the relative computational power of structures related to the ordered field of reals, specifically using the notion of generic Muchnik reducibility. We show that any expansion of the reals by a continuous function has no more computing power than the reals, answering a question of Igusa, Knight, and Schweber. On the other hand, we show that there is a certain Borel expansion of the reals that is strictly more powerful than the reals and such that any Borel quotient of the reals reduces to it.
(with Omer Mermelstein )
Journal of Symbolic Logic , Volume 86 , Issue 4 , December 2021 , pp. 1632 - 1656
Abstract:
We build a new spectrum of recursive models (
(with Omer Mermelstein )
Proceedings of the American Mathematics Society, 150 (2022), 381-395
Abstract:
We show that for a model complete strongly minimal theory whose pregeometry is flat, the recursive spectrum (SRM(
Combined with previous results, this leaves precisely 8 sets for which it is not yet determined whether they are spectra of a model complete strongly minimal theory with a flat pregeometry.
(with Matthew Harrison-Trainor, Noah D. Schweber )
Journal of Mathematical Logic, Vol. 21, No. 3 (2021)
Abstract:
We say that a theory
(with Joseph S. Miller, Mariya I. Soskova, Noah D. Schweber )
Advanced in Math, Volume 436 , January 2024, 109397
Abstract:
We study the relative computational complexity of expansions of Cantor and Baire space in terms of generic Muchnik reducibility. We show that no expansion of Cantor space by countably many unary or closed relations can give a generic Muchnik degree strictly between the degree of Cantor space and the degree of Baire space. Similarly, assuming
(with Matthew Harrison-Trainor, Meng-Che Ho )
International Journal of Algebra and Computation, Vol. 35, No. 02, pp. 311-327 (2025)
Abstract:
We answer two questions on the complexities of decision problems of groups, each related to a classical result. First, C.\ Miller characterized the complexity of the isomorphism problem for finitely presented groups in 1971. We do the same for the isomorphism problem for recursively presented groups. Second, the fact that every Turing degree appears as the degree of the word problem of a finitely presented group is shown independently by multiple people in the 1960s. We answer the analogous question for degrees of ceers instead of Turing degrees. We show that the set of ceers which are computably equivalent to a finitely presented group is
(with David Gonzalez, Steffen Lempp, Dino Rossegger, Hongyu Zhu )
Proceedings of the AMS, to appear
Abstract:
We investigate the descriptive complexity of the set of models of first-order theories. Using classical results of Knight and Solovay, we give a sharp condition for complete theories to have a
(with Meng-Che Ho )
Journal of Symbolic Logic, to appear
Abstract:
The study of the word problems of groups dates back to Dehn in 1911, and has been a central topic of study in both group theory and computability theory. As most naturally occurring presentations of groups are recursive, their word problems can be thought of as a computably enumerable equivalence relation (ceer). In this paper, we study the word problem of groups in the framework of ceer degrees, introducing a new metric with which to study word problems. This metric is more refined than the classical context of Turing degrees.
Classically, every Turing degree is realized as the word problem of some c.e.\ group, but this is not true for ceer degrees. This motivates us to look at the classical constructions and show that there is a group whose word problem is not universal, but becomes universal after taking any nontrivial free product, which we call *-universal. This shows that existing constructions of the Higman embedding theorem do not preserve ceer degrees. We also study the index set of various classes of groups defined by their properties as a ceer: groups whose word problems are dark (equivalently, algorithmically finite as defined by Miasnikov and Osin), universal, and *-universal groups.
Recursion Theory
(with Steffen Lempp, Joseph S. Miller, Keng Meng Ng, Luca San Mauro, Andrea Sorbi )
Journal of Symbolic Logic, 79 (2014), 60-88
Abstract:
We study computably enumerable equivalence relations (ceers) under the reducibility
(with Peter Gerdes, Joseph S. Miller )
Annals of Pure and Applied Logic, 165 (2014), no. 3, 803--811
Abstract:
We study the degrees of bi-hyperhyperimmune (bi-hhi) sets. Our main result characterizes these degrees as those that compute a function that is not dominated by any
(with Mingzhong Cai, David Diamondstone, Carl Jockusch, Steffen Lempp )
Fundamenta Mathematicae, 234 (2016), 41–53
Abstract:
Let
(with Andrea Sorbi )
Journal of Symbolic Logic, 81 (2016), 1375–1395
Abstract:
Let
(with Serikzhan Badaev, Andrea Sorbi )
A chapter in the book Computability and Complexity, in the Lecture Notes in Computer Science series, 418–451
Abstract:
We review the literature on universal computably enumerable equivalencerelations, i.e. the computably enumerable equivalence relations (ceers)which are
(with Rutger Kuyper, Steffen Lempp, Mariya I. Soskova, Mars M. Yamaleev )
A chapter in the book Computability and Complexity, in the Lecture Notes in Computer Science series, 547--562
Abstract:
In this paper, we show that the so-called "double bubbles" are not downward dense in the d.c.e. degrees. Here, a pair of d.c.e. degrees
(with Andrea Sorbi )
Annals of Pure and Applied Logic, 169 (2018), 243–259
Abstract:
We study computably enumerable equivalence relations (or, ceers), under computable reducibility
(with Mingzhong Cai, David Diamondstone, Noah D. Schweber )
Computability, vol. 12, no. 2, pp. 101-115, 2023
Abstract:
We study a class of operators on Turing degrees arising naturally from ultrafilters. Suppose
Our main result is that the converse also holds: if
(with Hristo A. Ganchev, Rutger Kuyper, Steffen Lempp, Joseph S. Miller, Alexandra A. Soskova, Mariya I. Soskova )
Transactions of the American Mathematical Society, 372 (2019), no. 3, 1631-1670.
Abstract:
A set
(with Andrea Sorbi )
Computability, vol. 8, no. 3-4, 193-241, 2019, Issue "Oberwolfach Workshop on Computability Theory 2018"
Abstract:
We study computably enumerable equivalence relations (abbreviated as ceers under computable reducibility, and we investigate the resulting degree structure
(with Gregory Igusa, Joseph S. Miller, Mariya I. Soskova )
Israel Journal of Mathematics, October 2019, Volume 234, Issue 2, pp 743–767
Abstract:
The continuous degrees measure the computability-theoretic content of elements of computable metric spaces. They properly extend the Turing degrees and naturally embed into the enumeration degrees. Although nontotal (i.e., non-Turing) continuous degrees exist, they are all very close to total: joining a continuous degree with a total degree that is not below it always results in a total degree. We call this property almost totality .
We prove that the almost total degrees coincide with the continuous degrees. Since the total degrees are definable in the partial order of enumeration degrees (citation), we see that the continuous degrees are also definable. Applying earlier work on the continuous degrees (citation), this shows that the relation "PA above" on the total degrees is definable in the enumeration degrees.
In order to prove that every almost total degree is continuous, we pass through another characterization of the continuous degrees that slightly simplifies one of Kihara and Pauly (citation). We prove that the enumeration degree of
(with Serikzhan Badaev )
Journal of Symbolic Logic, June 2019 , pp. 1-26
Abstract:
We examine how degrees of computably enumerable equivalence relations (ceers) under computable reduction break down into isomorphism classes. Two ceers are isomorphic if there is a computable permutation of
We conclude by answering some lingering open questions from the literature: Gao and Gerdes (citation) define the collection of essentially FC ceers to be those which are reducible to a ceer all of whose classes are finite. They show that the index set of essentially FC ceers is
Andrews and Sorbi examined strong minimal covers of downwards-closed sets of degrees of ceers. We show that if
Lastly, we show that there exists an infinite antichain of weakly precomplete ceers.
(with Noah D. Schweber, Andrea Sorbi )
Annals of Pure and Applied Logic, 171 (2020), no. 8, 102811, 23 pp.
Abstract:
We show that the theory of the partial order of computably enumerable equivalence relations (ceers) under computable reduction is 1-equivalent to true arithmetic. We show the same result for the structure comprised of the dark ceers and the structure comprised of the light ceers. We also show the same for the structure of
(with Noah D. Schweber, Andrea Sorbi )
Journal of Logic and Computation, Volume 30, Issue 3, April 2020, Pages 765–783
Abstract:
We examine the property of self-fullness of computably enumerablee quivalence relations (ceers) and its relation to the uniform join operation. We answer a question from ( citation ) by showing that there are self-full ceers
(with Andrea Sorbi )
Journal of Symbolic Logic, 87(3), 1260-1282
Abstract:
It is known that every non-universal self-full degree in the structure of the degrees of computably enumerable equivalence relations (ceers) under computable reducibility has exactly one strong minimal cover. This leaves little room for embedding wide partial orders as initial segments using self-full degrees. We show that considerably more can be done by staying entirely inside the collection of non-self-full degrees. We show that the poset
(with Steffen Lempp, Manat Mustafa, Noah D. Schweber )
Journal of Logic and Computation, Volume 32, Issue 1, January 2022, Pages 98–114
Abstract:
We generalize the analysis of Andrews, Schweber and Sorbi of the first-order theory of the partial order of degrees of c.e. equivalence relations to higher computability theory, specifically to the setting of a regular cardinal.
(with Peter Gerdes, Steffen Lempp, Joseph S. Miller, Noah D. Schweber )
Logic journal of the IGPL, Volume 30, Issue 3, June 2022, Pages 499–518
Abstract:
Combinatorial operations on sets are almost never well-defined on Turing degrees, a fact so obvious that counterexamples are worth exhibiting. The case we focus on is the symmetric difference operator; there are pairs of (nonzero) degrees for which the symmetric difference operation is well-defined. Some examples can be extracted from the literature, for example, from the existence of nonzero degrees with strong minimal covers. We focus on the case of incomparable r.e. degrees for which the symmetric difference operation is well-defined.
(with Daniel F. Belin, Luca San Mauro )
The Journal of Symbolic Logic , Volume 88 , Issue 3 , September 2023 , pp. 1038 - 1063
Abstract:
We examine the degree structure
(with Luca San Mauro )
Journal of Symbolic Logic. 2024;89(2):918-944.
Abstract:
We answer several questions about the computable Friedman-Stanley jump on equivalence relations. This jump, introduced by Clemens, Coskey, and Krakoff, deepens the natural connection between the study of computable reduction and its Borel analog studied deeply in descriptive set theory.
(with Luca San Mauro )
Proceedings of the AMS, To Appear
Abstract:
Coskey, Hamkins, and Miller [CHM] proposed two possible analogues of the class of countable Borel equivalence relations in the setting of computable reducibility of equivalence relations on the computably enumerable (c.e.) sets. The first is based on effectivizing the Lusin/Novikov theorem while the latter is based on effectivizing the Feldman/Moore theorem. They asked for an analysis of which degrees under computable reducibility are attained under each of these notions. We investigate these two notions, in particular showing that the latter notion has a strict dichotomy theorem: Every such equivalence relation is either equivalent to the relation of equality (
(with Steffen Lempp, Alberto Marcone, Joseph S. Miller, Manlio Valenti )
Computability, to appear.
Abstract:
A partial order
Model Theory
(with H. Jerome Keisler )
Journal of Symbolic Logic, 80 (2015) 1149-1181
Abstract:
Every complete first order theory has a corresponding complete theory in continuous logic, called the randomization theory. It has two sorts, a sort for random elements of models of the first order theory, and a sort for events. In this paper we establish connections between properties of countable models of a first order theory and corresponding properties of separable models of the randomization theory. We show that the randomization theory has a prime model if and only if the first order theory has a prime model. And the randomization theory has the same number of separable homogeneous models as the first order theory has countable homogeneous models. We also show that when
(with Isaac Goldbring, H. Jerome Keisler )
Annals of Pure and Applied Logic, Volume 166, Issue 3, 2015, 325-341
Abstract:
The randomization of a complete first order theory
(with Vincent Guingona )
Proceedings of the American Mathematical Society, 144 (2016), 2241-2256
Abstract:
We show VC-minimality is
(with Isaac Goldbring )
Semigroup Forum 2018, Volume 97, Issue 3, pp 471–477
Abstract:
Motivated by a question of Di Nasso, we show that Hindman's Theorem is equivalent to the existence of idempotent types in countable complete extensions of Peano Arithmetic.
(with Isaac Goldbring, H. Jerome Keisler )
Journal of Mathematical Logic, Vol. 19, No. 01, (2019)
Abstract:
The randomization of a complete first order theory
(with Gabriel Conant, Isaac Goldbring )
Journal of Group Theory, Volume 22 (2019), 63-82
Abstract:
We consider the question of when sets definable in first-order expansions of groups contain the product of two infinite sets (we refer to this as the "productset property"). We show that the productset property holds for any definable subset
(with Isaac Goldbring, Sherwood Hachtman, H. Jerome Keisler, David Marker )
Archive for Mathematical Logic volume 59, pages743–754(2020)
Abstract:
In the paper "Randomizations of Scattered Sentences", Keisler showed that if Martin’s axiom for
Other
(with Mingzhong Cai, David Diamondstone, Steffen Lempp, Joseph S. Miller )
Israel Journal of Mathematics, 207 (2015), 449-478
Abstract:
Cai introduced the provability degrees as the degree structure of the computable algorithms under the reduction
(with Andrea Sorbi )
The Review of Symbolic Logic, 14(4), 838-865
Abstract:
We study effectively inseparable (abbreviated as e.i.) pre-lattices (i.e.structures of the form