{"title":"Constructing models of small ordered theories with maximal countable spectrum","authors":"B. Baizhanov , T. Zambarnaya","doi":"10.1016/j.apal.2025.103659","DOIUrl":"10.1016/j.apal.2025.103659","url":null,"abstract":"<div><div>We propose a method for the construction of countable models of small theories. We then apply it to prove theorems concerning the maximal number of countable non-isomorphic models of linearly ordered theories.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 2","pages":"Article 103659"},"PeriodicalIF":0.6,"publicationDate":"2026-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145242574","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Proof-theoretic investigation of λβ-reduction in the simply typed λ-calculus","authors":"William Stirton","doi":"10.1016/j.apal.2025.103657","DOIUrl":"10.1016/j.apal.2025.103657","url":null,"abstract":"<div><div>The paper defines a function <em>f</em> from simply typed <em>λ</em>-terms to natural numbers and proves that, if <span><math><msup><mrow><mi>M</mi></mrow><mrow><mo>⁎</mo></mrow></msup></math></span> is a simply typed <em>λ</em>-term formed by contracting an arbitrary <em>λβ</em>-redex within another term <em>M</em>, then <span><math><mi>f</mi><mo>(</mo><msup><mrow><mi>M</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mo>)</mo><mo><</mo><mi>f</mi><mo>(</mo><mi>M</mi><mo>)</mo></math></span>. Unlike previous proofs of similar theorems, the redex contracted may be completely arbitrary, i.e. without any restriction on rule (<em>ξ</em>). The function <em>f</em> itself is related to, and no more computationally difficult than, a similar-looking function defined in Schütte's <em>Proof Theory</em> (1977).</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103657"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145157519","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Unified inverse correspondence for LE-logics","authors":"Alessandra Palmigiano , Mattia Panettiere","doi":"10.1016/j.apal.2025.103635","DOIUrl":"10.1016/j.apal.2025.103635","url":null,"abstract":"<div><div>We generalize Kracht's theory of internal describability from classical modal logic to the family of all logics canonically associated with varieties of normal lattice expansions (LE algebras). We work in the purely algebraic setting of perfect LEs; the formulas playing the role of Kracht's formulas in this generalized setting pertain to a first order language whose atoms are special inequalities between terms of perfect algebras. Via duality, formulas in this language can be equivalently translated into first order conditions in the frame correspondence languages of several types of relational semantics for LE-logics.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103635"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144694400","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Fundamental sequences based on localization","authors":"Gunnar Wilken","doi":"10.1016/j.apal.2025.103658","DOIUrl":"10.1016/j.apal.2025.103658","url":null,"abstract":"<div><div>Building on Buchholz' assignment for ordinals below Bachmann-Howard ordinal, see <span><span>[2]</span></span>, we introduce systems of fundamental sequences for two kinds of relativized <em>ϑ</em>-function-based notation systems of strength <span><math><msubsup><mrow><mi>Π</mi></mrow><mrow><mn>1</mn></mrow><mrow><mn>1</mn></mrow></msubsup><msub><mrow><mi>-CA</mi></mrow><mrow><mn>0</mn></mrow></msub></math></span> and prove Bachmann property for these systems, which is essential for monotonicity properties of subrecursive hierarchies defined on the basis of fundamental sequences. The central notion of our construction is the notion of <em>localization</em>, which was introduced in <span><span>[12]</span></span>.</div><div>The first kind of <em>stepwise defined ϑ</em>-functions over ordinal addition as basic function fits the framework of the ordinal arithmetical toolkit developed in <span><span>[12]</span></span>, whereas the second kind of <em>ϑ</em>-functions is defined <em>simultaneously</em> and will allow for further generalization to larger proof-theoretic ordinals, see <span><span>[10]</span></span>.</div><div>The systems of fundamental sequences given here enable the investigation of fundamental sequences and independence phenomena also in the context of patterns of resemblance, an approach to ordinal notations that is both semantic and combinatorial and was first introduced by Carlson in <span><span>[4]</span></span> and further analyzed in <span><span>[11]</span></span>, <span><span>[13]</span></span>, <span><span>[14]</span></span>, <span><span>[5]</span></span>.</div><div>Our exposition is put into the context of the abstract approach to fundamental sequences developed by Buchholz, Cichon, and Weiermann in <span><span>[3]</span></span>. The results of this paper will be applied to the theory of Goodstein sequences, extending results of <span><span>[7]</span></span>.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103658"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145219605","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Countable ordered groups and Weihrauch reducibility","authors":"Ang Li","doi":"10.1016/j.apal.2025.103644","DOIUrl":"10.1016/j.apal.2025.103644","url":null,"abstract":"<div><div>This paper continues to study the connection between reverse mathematics and Weihrauch reducibility. In particular, we study the problems formed from Maltsev's theorem <span><span>[11]</span></span> on the order types of countable ordered groups. Solomon <span><span>[14]</span></span> showed that the theorem is equivalent to <span><math><msubsup><mrow><mi>Π</mi></mrow><mrow><mn>1</mn></mrow><mrow><mn>1</mn></mrow></msubsup></math></span>-<span><math><mi>C</mi><msub><mrow><mi>A</mi></mrow><mrow><mn>0</mn></mrow></msub></math></span>, the strongest of the big five subsystems of second order arithmetic. We show that the strength of the theorem comes from having a dense linear order without endpoints in its order type. Then, we show that for the related Weihrauch problem to be strong enough to be equivalent to <span><math><mover><mrow><mi>WF</mi></mrow><mrow><mo>ˆ</mo></mrow></mover></math></span> (the analog problem of <span><math><msubsup><mrow><mi>Π</mi></mrow><mrow><mn>1</mn></mrow><mrow><mn>1</mn></mrow></msubsup></math></span>-<span><math><mi>C</mi><msub><mrow><mi>A</mi></mrow><mrow><mn>0</mn></mrow></msub></math></span>), an order-preserving function is necessary in the output. Without the order-preserving function, the problems are very much to the side compared to analog problems of the big five.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103644"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144771199","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Nikolay Bazhenov , Vittorio Cipriani , Sanjay Jain , Luca San Mauro , Frank Stephan
{"title":"Classifying different criteria for learning algebraic structures","authors":"Nikolay Bazhenov , Vittorio Cipriani , Sanjay Jain , Luca San Mauro , Frank Stephan","doi":"10.1016/j.apal.2025.103648","DOIUrl":"10.1016/j.apal.2025.103648","url":null,"abstract":"<div><div>In the last years there has been a growing interest in the study of learning problems associated with algebraic structures. The framework we use models the scenario in which a learner is given larger and larger fragments of a structure from a given target family and is required to output an hypothesis about the structure's isomorphism type. So far researchers focused on <strong>Ex</strong>-learning, in which the learner is asked to eventually stabilize to the correct hypothesis, and on restrictions where the learner is allowed to change the hypothesis a fixed number of times. Yet, other learning paradigms coming from classical algorithmic learning theory remained unexplored. We study the ‘‘learning power’’ of such criteria, comparing them via descriptive-set-theoretic tools thanks to the novel notion of <em>E</em>-learnability. The main outcome of this paper is that such criteria admit natural syntactic characterizations in terms of infinitary formulas analogous to the one given for <strong>Ex</strong>-learning in <span><span>[8]</span></span>. Such characterizations give a powerful method to understand whether a family of structures is learnable with respect to the desired criterion.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103648"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144903024","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Thick sets and the Central Set Theorem","authors":"Teng Zhang","doi":"10.1016/j.apal.2025.103645","DOIUrl":"10.1016/j.apal.2025.103645","url":null,"abstract":"<div><div>In 1981, Furstenberg introduced the notion of central sets, and he established the Central Set Theorem. Since then, several generalizations of this result have been found, where a significant version is obtained by De, Hindman and Strauss. In this article, we find that the Central Set Theorem can be improved further. And we observe that there are some connections between thick sets and <em>J</em>-sets. Based on that, we establish a CST-type result for thick sets. Moreover, we introduce a new notion called super thick sets, and find that this notion has rich combinatorial properties. In particular, it contains additive and multiplicative structures, and it has a CST-type result for two operations. In addition, it can be partitioned into <em>κ</em> super thick subsets in very weakly cancellative weak rings with size <em>κ</em>.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103645"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144723034","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Description of structures computable in polynomial time","authors":"Pavel E. Alaev","doi":"10.1016/j.apal.2025.103636","DOIUrl":"10.1016/j.apal.2025.103636","url":null,"abstract":"<div><div>We describe structures computable in polynomial time (P-computable) by a simple and short criterion. We prove that every substructure of a P-computable structure generated by a c.e. set also has a P-computable presentation. For example, we easily prove that every computable torsion-free Abelian group has a P-computable presentation.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103636"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144703217","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ali Madanshekaf , Adam Přenosil , Zeinab Khanjanzadeh Seresti , Constantine Tsinakis
{"title":"Equivalence of multiset-based consequence relations","authors":"Ali Madanshekaf , Adam Přenosil , Zeinab Khanjanzadeh Seresti , Constantine Tsinakis","doi":"10.1016/j.apal.2025.103646","DOIUrl":"10.1016/j.apal.2025.103646","url":null,"abstract":"<div><div>The pioneering work of Blok and Jónsson, and its further development by Galatos and Tsinakis, initiated an abstract study of consequence relations through the lens of module theory, treating consequence relations over all types of syntactic objects on an equal footing. Despite this generality, their framework retains the assumption that premises in a consequence relation form a mere set, rather than a more structured collection. An attempt to extend this framework to account for inferentially substructural generalizations of consequence relations, where the premises have the structure of a finite multiset, was recently made by Cintula, Gil-Férez, Moraschini, and Paoli. In this paper, we propose a different substructural generalization of the Galatos–Tsinakis approach, where the premises are instead taken to form a set of finite multisets. This yields a smoother and more flexible framework that, unlike the approach of Cintula et al., subsumes the original theory of Galatos and Tsinakis as a special case.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103646"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144771198","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Uncountable homogeneous structures","authors":"Adam Bartoš, Wiesław Kubiś","doi":"10.1016/j.apal.2025.103649","DOIUrl":"10.1016/j.apal.2025.103649","url":null,"abstract":"<div><div>We study the existence of uncountable first-order structures that are homogeneous with respect to their finitely generated substructures. In many classical cases this is either well-known or follows from general facts, for example, if the language is finite and relational then ultrapowers provide arbitrarily large such structures. On the other hand, there are no general results saying that uncountable homogeneous structures with a given age exist. We examine the monoid of self-embeddings of a fixed countable homogeneous structure and, using abstract Fraïssé theory, we present a method of constructing an uncountable homogeneous structure, based on the amalgamation property of this monoid.</div></div>","PeriodicalId":50762,"journal":{"name":"Annals of Pure and Applied Logic","volume":"177 1","pages":"Article 103649"},"PeriodicalIF":0.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144908766","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}