Search results for 'definability' (try it on Scholar)

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  1.  26
    Juha Kontinen & Jakub Szymanik (2011). Characterizing Definability of Second-Order Generalized Quantifiers. In L. Beklemishev & R. de Queiroz (eds.), Proceedings of the 18th Workshop on Logic, Language, Information and Computation, Lecture Notes in Artificial Intelligence 6642. Springer
    We study definability of second-order generalized quantifiers. We show that the question whether a second-order generalized quantifier $\sQ_1$ is definable in terms of another quantifier $\sQ_2$, the base logic being monadic second-order logic, reduces to the question if a quantifier $\sQ^{\star}_1$ is definable in $\FO(\sQ^{\star}_2,<,+,\times)$ for certain first-order quantifiers $\sQ^{\star}_1$ and $\sQ^{\star}_2$. We use our characterization to show new definability and non-definability results for second-order generalized quantifiers. In particular, we show that the monadic second-order majority quantifier $\most^1$ (...)
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  2. Larisa Maksimova (2006). Definability and Interpolation in Non-Classical Logics. Studia Logica 82 (2):271 - 291.
    Algebraic approach to study of classical and non-classical logical calculi was developed and systematically presented by Helena Rasiowa in [48], [47]. It is very fruitful in investigation of non-classical logics because it makes possible to study large families of logics in an uniform way. In such research one can replace logics with suitable classes of algebras and apply powerful machinery of universal algebra. In this paper we present an overview of results on interpolation and definability in modal and positive (...)
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  3.  3
    Vincent Guingona (2012). On Uniform Definability of Types Over Finite Sets. Journal of Symbolic Logic 77 (2):499-514.
    In this paper, using definability of types over indiscernible sequences as a template, we study a property of formulas and theories called "uniform definability of types over finite sets" (UDTFS). We explore UDTFS and show how it relates to well-known properties in model theory. We recall that stable theories and weakly o-minimal theories have UDTFS and UDTFS implies dependence. We then show that all dp-minimal theories have UDTFS.
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  4.  30
    Tapani Hyttinen & Gabriel Sandu (2000). Henkin Quantifiers and the Definability of Truth. Journal of Philosophical Logic 29 (5):507-527.
    Henkin quantifiers have been introduced in Henkin (1961). Walkoe (1970) studied basic model-theoretical properties of an extension $L_{*}^{1}$ (H) of ordinary first-order languages in which every sentence is a first-order sentence prefixed with a Henkin quantifier. In this paper we consider a generalization of Walkoe's languages: we close $L_{*}^{1}$ (H) with respect to Boolean operations, and obtain the language L¹(H). At the next level, we consider an extension $L_{*}^{2}$ (H) of L¹(H) in which every sentence is an L¹(H)-sentence prefixed with (...)
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  5.  9
    Rohit Parikh (2011). Beth Definability, Interpolation and Language Splitting. Synthese 179 (2):211 - 221.
    Both the Beth definability theorem and Craig's lemma (interpolation theorem from now on) deal with the issue of the entanglement of one language L1 with another language L2, that is to say, information transfer—or the lack of such transfer—between the two languages. The notion of splitting we study below looks into this issue. We briefly relate our own results in this area as well as the results of other researchers like Kourousias and Makinson, and Peppas, Chopra and Foo.Section 3 (...)
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  6.  6
    Stanislav O. Speranski (2013). A Note on Definability in Fragments of Arithmetic with Free Unary Predicates. Archive for Mathematical Logic 52 (5-6):507-516.
    We carry out a study of definability issues in the standard models of Presburger and Skolem arithmetics (henceforth referred to simply as Presburger and Skolem arithmetics, for short, because we only deal with these models, not the theories, thus there is no risk of confusion) supplied with free unary predicates—which are strongly related to definability in the monadic SOA (second-order arithmetic) without × or + , respectively. As a consequence, we obtain a very direct proof for ${\Pi^1_1}$ -completeness (...)
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  7.  5
    T. Lee (2003). Arithmetical Definability Over Finite Structures. Mathematical Logic Quarterly 49 (4):385.
    Arithmetical definability has been extensively studied over the natural numbers. In this paper, we take up the study of arithmetical definability over finite structures, motivated by the correspondence between uniform AC0 and FO. We prove finite analogs of three classic results in arithmetical definability, namely that < and TIMES can first-order define PLUS, that < and DIVIDES can first-order define TIMES, and that < and COPRIME can first-order define TIMES. The first result sharpens the equivalence FO =FO (...)
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  8.  18
    Ernst Zimmermann (2003). Elementary Definability and Completeness in General and Positive Modal Logic. Journal of Logic, Language and Information 12 (1):99-117.
    The paper generalises Goldblatt's completeness proof for Lemmon–Scott formulas to various modal propositional logics without classical negation and without ex falso, up to positive modal logic, where conjunction and disjunction, andwhere necessity and possibility are respectively independent.Further the paper proves definability theorems for Lemmon–Scottformulas, which hold even in modal propositional languages without negation and without falsum. Both, the completeness theorem and the definability theoremmake use only of special constructions of relations,like relation products. No second order logic, (...)
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  9.  3
    Iain A. Stewart (1997). Regular Subgraphs in Graphs and Rooted Graphs and Definability in Monadic Second‐Order Logic. Mathematical Logic Quarterly 43 (1):1-21.
    We investigate the definability in monadic ∑11 and monadic Π11 of the problems REGk, of whether there is a regular subgraph of degree k in some given graph, and XREGk, of whether, for a given rooted graph, there is a regular subgraph of degree k in which the root has degree k, and their restrictions to graphs in which every vertex has degree at most k, namely REGkk and XREGkk, respectively, for k ≥ 2 . Our motivation partly stems (...)
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  10. Alexandra Shlapentokh (2001). Diophantine Definability Over Non-Finitely Generated Non-Degenerate Modules of Algebraic Extensions of ℚ. Archive for Mathematical Logic 40 (4):297-328.
    We investigate the issues of Diophantine definability over the non-finitely generated version of non-degenerate modules contained in the infinite algebraic extensions of the rational numbers. In particular, we show the following. Let k be a number field and let K inf be a normal algebraic, possibly infinite, extension of k such that k has a normal extension L linearly disjoint from K inf over k. Assume L is totally real and K inf is totally complex. Let M inf be (...)
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  11. Bruno Teheux (forthcoming). Modal Definability Based on Łukasiewicz Validity Relations. Studia Logica:1-21.
    We study two notions of definability for classes of relational structures based on modal extensions of Łukasiewicz finitely-valued logics. The main results of the paper are the equivalent of the Goldblatt-Thomason theorem for these notions of definability.
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  12.  3
    Jon Barwise (1975). Admissible Sets and Structures: An Approach to Definability Theory. Springer-Verlag.
  13.  14
    N. C. A. Da Costa & A. A. M. Rodrigues (2007). Definability and Invariance. Studia Logica 86 (1):1-30.
    In his thesis Para uma Teoria Geral dos Homomorfismos (1944), the Portuguese mathematician José Sebastião e Silva constructed an abstract or generalized Galois theory, that is intimately linked to F. Klein’s Erlangen Program and that foreshadows some notions and results of today’s model theory; an analogous theory was independently worked out by M. Krasner in 1938. In this paper, we present a version of the theory making use of tools which were not at Silva’s disposal. At the same time, we (...)
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  14.  13
    Hajnal Andréka, Judit X. Madarász & István Németi (2005). Mutual Definability Does Not Imply Definitional Equivalence, a Simple Example. Mathematical Logic Quarterly 51 (6):591-597.
    We give two theories, Th1 and Th2, which are explicitly definable over each other , but are not definitionally equivalent. The languages of the two theories are disjoint.
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  15.  6
    David Fernández Duque (2011). On the Modal Definability of Simulability by Finite Transitive Models. Studia Logica 98 (3):347-373.
    We show that given a finite, transitive and reflexive Kripke model 〈 W , ≼, ⟦ ⋅ ⟧ 〉 and $${w \in W}$$ , the property of being simulated by w (i.e., lying on the image of a literalpreserving relation satisfying the ‘forth’ condition of bisimulation) is modally undefinable within the class of S4 Kripke models. Note the contrast to the fact that lying in the image of w under a bi simulation is definable in the standard modal language even (...)
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  16. Veikko Rantala (1977). Aspects of Definability.
     
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  17.  18
    Lauri Hella, Jouko Väänänen & Dag Westerståhl (1997). Definability of Polyadic Lifts of Generalized Quantifiers. Journal of Logic, Language and Information 6 (3):305-335.
    We study generalized quantifiers on finite structures.With every function : we associate a quantifier Q by letting Q x say there are at least (n) elementsx satisfying , where n is the sizeof the universe. This is the general form ofwhat is known as a monotone quantifier of type .We study so called polyadic liftsof such quantifiers. The particular lifts we considerare Ramseyfication, branching and resumption.In each case we get exact criteria fordefinability of the lift in terms of simpler quantifiers.
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  18.  2
    S. Barry Cooper (2015). The Machine as Data: A Computational View of Emergence and Definability. Synthese 192 (7):1955-1988.
    Turing’s paper on computable numbers has played its role in underpinning different perspectives on the world of information. On the one hand, it encourages a digital ontology, with a perceived flatness of computational structure comprehensively hosting causality at the physical level and beyond. On the other, it can give an insight into the way in which higher order information arises and leads to loss of computational control—while demonstrating how the control can be re-established, in special circumstances, via suitable type reductions. (...)
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  19.  7
    Olivier Chapuis & Pascal Koiran (1999). Definability of Geometric Properties in Algebraically Closed Fields. Mathematical Logic Quarterly 45 (4):533-550.
    We prove that there exists no sentence F of the language of rings with an extra binary predicat I2 satisfying the following property: for every definable set X ⊆ ℂ2, X is connected if and only if ⊧ F, where I2 is interpreted by X. We conjecture that the same result holds for closed subset of ℂ2. We prove some results motivated by this conjecture.
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  20.  7
    Akito Tsuboi & Saharon Shelah (2002). Definability of Initial Segments. Notre Dame Journal of Formal Logic 43 (2):65-73.
    In any nonstandard model of Peano arithmetic, the standard part is not first-order definable. But we show that in some model the standard part is definable as the unique solution of a formula , where P is a unary predicate variable.
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  21.  5
    Hassan Sfouli (2010). Definability and Nondefinability Results for Certain o-Minimal Structures. Mathematical Logic Quarterly 56 (5):503-507.
    The main goal of this note is to study for certain o-minimal structures the following propriety: for each definable C∞ function g0: [0, 1] → ℝ there is a definable C∞ function g: [–ε, 1] → ℝ, for some ε > 0, such that g = g0 for all x ∈ [0, 1].
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  22.  10
    Shih Ping Tung (1992). Arithmetic Definability by Formulas with Two Quantifiers. Journal of Symbolic Logic 57 (1):1-11.
    We give necessary conditions for a set to be definable by a formula with a universal quantifier and an existential quantifier over algebraic integer rings or algebraic number fields. From these necessary conditions we obtain some undefinability results. For example, N is not definable by such a formula over Z. This extends a previous result of R. M. Robinson.
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  23.  3
    Kenji Fukuzaki (2012). Definability of the Ring of Integers in Some Infinite Algebraic Extensions of the Rationals. Mathematical Logic Quarterly 58 (4‐5):317-332.
    Let K be an infinite Galois extension of the rationals such that every finite subextension has odd degree over the rationals and its prime ideals dividing 2 are unramified. We show that its ring of integers is first-order definable in K. As an application we prove that equation image together with all its Galois subextensions are undecidable, where Δ is the set of all the prime integers which are congruent to −1 modulo 4.
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  24.  2
    Larisa Maksimova (2011). Interpolation and Definability Over the Logic Gl. Studia Logica 99 (1-3):249-267.
    In a previous paper [ 21 ] all extensions of Johansson’s minimal logic J with the weak interpolation property WIP were described. It was proved that WIP is decidable over J. It turned out that the weak interpolation problem in extensions of J is reducible to the same problem over a logic Gl, which arises from J by adding tertium non datur. In this paper we consider extensions of the logic Gl. We prove that only finitely many logics over Gl (...)
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  25. Akito Tsuboi (1992). On Definability of Normal Subgroups of a Superstable Group. Mathematical Logic Quarterly 38 (1):101-106.
    In this note we treat maximal and minimal normal subgroups of a superstable group and prove that these groups are definable under certain conditions. Main tool is a superstable version of Zil'ber's indecomposability theorem.
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  26. Guy McCusker (1997). Games and Definability for FPC. Bulletin of Symbolic Logic 3 (3):347-362.
    A new games model of the language FPC, a type theory with products, sums, function spaces and recursive types, is described. A definability result is proved, showing that every finite element of the model is the interpretation of some term of the language.
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  27. Kentaro Fujimoto (2010). Relative Truth Definability of Axiomatic Truth Theories. Bulletin of Symbolic Logic 16 (3):305-344.
    The present paper suggests relative truth definability as a tool for comparing conceptual aspects of axiomatic theories of truth and gives an overview of recent developments of axiomatic theories of truth in the light of it. We also show several new proof-theoretic results via relative truth definability including a complete answer to the conjecture raised by Feferman in [13].
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  28.  3
    Balder Ten Cate, David Gabelaia & Dmitry Sustretov (2009). Modal Languages for Topology: Expressivity and Definability. Annals of Pure and Applied Logic 159 (1):146-170.
    In this paper we study the expressive power and definability for modal languages interpreted on topological spaces. We provide topological analogues of the van Benthem characterization theorem and the Goldblatt–Thomason definability theorem in terms of the well-established first-order topological language.
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  29.  5
    Eva Hoogland (2000). Algebraic Characterizations of Various Beth Definability Properties. Studia Logica 65 (1):91-112.
    In this paper it will be shown that the Beth definability property corresponds to surjectiveness of epimorphisms in abstract algebraic logic. This generalizes a result by I. Németi (cf. [11, Theorem 5.6.10]). Moreover, an equally general characterization of the weak Beth property will be given. This gives a solution to Problem 14 in [20]. Finally, the characterization of the projective Beth property for varieties of modal algebras by L. Maksimova (see [15]) will be shown to hold for the larger (...)
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  30.  10
    Eva Hoogland & Maarten Marx (2002). Interpolation and Definability in Guarded Fragments. Studia Logica 70 (3):373 - 409.
    The guarded fragment (GF) was introduced by Andréka, van Benthem and Németi as a fragment of first order logic which combines a great expressive power with nice, modal behavior. It consists of relational first order formulas whose quantifiers are relativized by atoms in a certain way. Slightly generalizing the admissible relativizations yields the packed fragment (PF). In this paper we investigate interpolation and definability in these fragments. We first show that the interpolation property of first order logic fails in (...)
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  31.  24
    Leo Harrington & Robert I. Soare (1996). Definability, Automorphisms, and Dynamic Properties of Computably Enumerable Sets. Bulletin of Symbolic Logic 2 (2):199-213.
    We announce and explain recent results on the computably enumerable (c.e.) sets, especially their definability properties (as sets in the spirit of Cantor), their automorphisms (in the spirit of Felix Klein's Erlanger Programm), their dynamic properties, expressed in terms of how quickly elements enter them relative to elements entering other sets, and the Martin Invariance Conjecture on their Turing degrees, i.e., their information content with respect to relative computability (Turing reducibility).
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  32.  98
    Adonai Sant'Anna, The Definability of Physical Concepts.
    Our main purpose here is to make some considerations about the definability of physical concepts like mass, force, time, space, spacetime, and so on. Our starting motivation is a collection of supposed definitions of closed system in the literature of physics and philosophy of physics. So, we discuss the problem of definitions in theoretical physics from the point of view of modern theories of definition. One of our main conclusions is that there are different kinds of definitions in physics (...)
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  33.  2
    Chris Pollett (1999). Structure and Definability in General Bounded Arithmetic Theories. Annals of Pure and Applied Logic 100 (1-3):189-245.
    The bounded arithmetic theories R2i, S2i, and T2i are closely connected with complexity theory. This paper is motivated by the questions: what are the Σi+1b-definable multifunctions of R2i? and when is one theory conservative over another? To answer these questions we consider theories , and where induction is restricted to prenex formulas. We also define which has induction up to the 0 or 1-ary L2-terms in the set τ. We show and and for . We show that the -multifunctions of (...)
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  34.  29
    M. Aiguier & F. Barbier (2007). An Institution-Independent Proof of the Beth Definability Theorem. Studia Logica 85 (3):333 - 359.
    A few results generalizing well-known classical model theory ones have been obtained in institution theory these last two decades (e.g. Craig interpolation, ultraproduct, elementary diagrams). In this paper, we propose a generalized institution-independent version of the Beth definability theorem.
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  35.  3
    Juha Kontinen (2004). Definability of Second Order Generalized Quantifiers. Dissertation,
    We study second order generalized quantifiers on finite structures. One starting point of this research has been the notion of definability of Lindström quantifiers. We formulate an analogous notion for second order generalized quantifiers and study definability of second order generalized quantifiers in terms of Lindström quantifiers.
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  36.  4
    J. W. Addison (2004). Tarski's Theory of Definability: Common Themes in Descriptive Set Theory, Recursive Function Theory, Classical Pure Logic, and Finite-Universe Logic. Annals of Pure and Applied Logic 126 (1-3):77-92.
    Although the theory of definability had many important antecedents—such as the descriptive set theory initiated by the French semi-intuitionists in the early 1900s—the main ideas were first laid out in precise mathematical terms by Alfred Tarski beginning in 1929. We review here the basic notions of languages, explicit definability, and grammatical complexity, and emphasize common themes in the theories of definability for four important languages underlying, respectively, descriptive set theory, recursive function theory, classical pure logic, and finite-universe (...)
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  37.  20
    Joseph Y. Halpern, Dov Samet & Ella Segev (2009). On Definability in Multimodal Logic. Review of Symbolic Logic 2 (3):451-468.
    Three notions of definability in multimodal logic are considered. Two are analogous to the notions of explicit definability and implicit definability introduced by Beth in the context of first-order logic. However, while by Beth’s theorem the two types of definability are equivalent for first-order logic, such an equivalence does not hold for multimodal logics. A third notion of definability, reducibility, is introduced; it is shown that in multimodal logics, explicit definability is equivalent to the (...)
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  38.  6
    Xavier Caicedo (2004). Definability and Automorphisms in Abstract Logics. Archive for Mathematical Logic 43 (8):937-945.
    In any model theoretic logic, Beth’s definability property together with Feferman-Vaught’s uniform reduction property for pairs imply recursive compactness, and the existence of models with infinitely many automorphisms for sentences having infinite models. The stronger Craig’s interpolation property plus the uniform reduction property for pairs yield a recursive version of Ehrenfeucht-Mostowski’s theorem. Adding compactness, we obtain the full version of this theorem. Various combinations of definability and uniform reduction relative to other logics yield corresponding results on the existence (...)
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  39.  8
    Keith Simmons (1994). A Paradox of Definability: Richard'S and poincaré'S Ways Out. History and Philosophy of Logic 15 (1):33-44.
    In 1905, Richard discovered his paradox of definability, and in a letter written that year he presented both the paradox and a solution to it.Soon afterwards, Poincaré endorsed a variant of Richard?s solution.In this paper, I critically examine Richard?s and Poincaré?s ways out.I draw on an objection of Peano?s, and argue that their stated solutions do not work.But I also claim that their writings suggest another way out, different from their stated solutions, and different from the orthodox Tarskian approach.I (...)
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  40.  11
    Andrei S. Morozov & Margarita V. Korovina (2008). On Σ‐Definability Without Equality Over the Real Numbers. Mathematical Logic Quarterly 54 (5):535-544.
    In [5] it has been shown that for first-order definability over the reals there exists an effective procedure which by a finite formula with equality defining an open set produces a finite formula without equality that defines the same set. In this paper we prove that there exists no such procedure for Σ-definability over the reals. We also show that there exists even no uniform effective transformation of the definitions of Σ-definable sets into new definitions of Σ-definable sets (...)
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  41. Valentin Goranko & Alberto Zanardo (2007). From Linear to Branching-Time Temporal Logics: Transfer of Semantics and Definability. Logic Journal of the Igpl 15 (1):53-76.
    This paper investigates logical aspects of combining linear orders as semantics for modal and temporal logics, with modalities for possible paths, resulting in a variety of branching time logics over classes of trees. Here we adopt a unified approach to the Priorean, Peircean and Ockhamist semantics for branching time logics, by considering them all as fragments of the latter, obtained as combinations, in various degrees, of languages and semantics for linear time with a modality for possible paths. We then consider (...)
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  42.  9
    Xavier Caicedo (1990). Definability Properties and the Congruence Closure. Archive for Mathematical Logic 30 (4):231-240.
    We introduce a natural class of quantifiersTh containing all monadic type quantifiers, all quantifiers for linear orders, quantifiers for isomorphism, Ramsey type quantifiers, and plenty more, showing that no sublogic ofL ωω (Th) or countably compact regular sublogic ofL ∞ω (Th), properly extendingL ωω , satisfies the uniform reduction property for quotients. As a consequence, none of these logics satisfies eitherΔ-interpolation or Beth's definability theorem when closed under relativizations. We also show the failure of both properties for any sublogic (...)
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  43.  19
    Ítala M. L. D'Ottaviano (1987). Definability and Quantifier Elimination for J3-Theories. Studia Logica 46 (1):37 - 54.
    The Joint Non-Trivialization Theorem, two Definability Theorems and the generalized Quantifier Elimination Theorem are proved for J 3-theories. These theories are three-valued with more than one distinguished truth-value, reflect certain aspects of model type logics and can. be paraconsistent. J 3-theories were introduced in the author's doctoral dissertation.
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  44.  11
    Marco Hollenberg (1998). Characterizations of Negative Definability in Modal Logic. Studia Logica 60 (3):357-386.
    Negative definability ([18]) is an alternative way of defining classes of Kripke frames via a modal language, one that enables us, for instance, to define the class of irreflexive frames. Besides a list of closure conditions for negatively definable classes, the paper contains two main theorems. First, a characterization is given of negatively definable classes of (rooted) finite transitive Kripke frames and of such classes defined using both traditional (positive) and negative definitions. Second, we characterize the negatively definable classes (...)
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  45.  19
    Gabriel Sandu (2000). Minimalism and the Definability of Truth. The Proceedings of the Twentieth World Congress of Philosophy 2000:143-153.
    In this paper I am going to inquire to what extent the main requirements of a minimalist theory of truth and falsity (as formulated, for example, by Horwich and Field) can be consistently implemented in a formal theory. I will discuss several of the existing logical theories of truth, including Tarski-type (un)definability results, Kripke’s partial interpretation of truth and falsity, Barwise and Moss’ theory based upon non-well-founded sets, McGee’s treatment of truth as a vague predicate, and Hintikka’s languages of (...)
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  46.  12
    Anand Pillay (1994). Definability of Types, and Pairs of o-Minimal Structures. Journal of Symbolic Logic 59 (4):1400-1409.
    Let T be a complete O-minimal theory in a language L. We first give an elementary proof of the result (due to Marker and Steinhorn) that all types over Dedekind complete models of T are definable. Let L * be L together with a unary predicate P. Let T * be the L * -theory of all pairs (N, M), where M is a Dedekind complete model of T and N is an |M| + -saturated elementary extension of N (and (...)
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  47.  25
    Lloyd Humberstone (1998). Note on Supervenience and Definability. Notre Dame Journal of Formal Logic 39 (2):243-252.
    The idea of a property's being supervenient on a class of properties is familiar from much philosophical literature. We give this idea a linguistic turn by converting it into the idea of a predicate symbol's being supervenient on a set of predicate symbols relative to a (first order) theory. What this means is that according to the theory, any individuals differing in respect to whether the given predicate applies to them also differ in respect to the application of at least (...)
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  48.  7
    Byunghan Kim & Rahim Moosa (2007). Stable Definability and Generic Relations. Journal of Symbolic Logic 72 (4):1163 - 1176.
    An amalgamation base p in a simple theory is stably definable if its canonical base is interdefinable with the set of canonical parameters for the ϕ-definitions of p as ϕ ranges through all stable formulae. A necessary condition for stably definability is given and used to produce an example of a supersimple theory with stable forking having types that are not stably definable. This answers negatively a question posed in [8]. A criterion for and example of a stably definable (...)
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  49.  10
    Michael Benedikt & H. Jerome Keisler (2003). Definability with a Predicate for a Semi-Linear Set. Journal of Symbolic Logic 68 (1):319-351.
    We settle a number of questions concerning definability in first order logic with an extra predicate symbol ranging over semi-linear sets. We give new results both on the positive and negative side: we show that in first-order logic one cannot query a semi-linear set as to whether or not it contains a line, or whether or not it contains the line segment between two given points. However, we show that some of these queries become definable if one makes (...)
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  50.  3
    Anand Pillay (1998). Definability and Definable Groups in Simple Theories. Journal of Symbolic Logic 63 (3):788-796.
    We continue the study of simple theories begun in [3] and [5]. We first find the right analogue of definability of types. We then develop the theory of generic types and stabilizers for groups definable in simple theories. The general ideology is that the role of formulas (or definability) in stable theories is replaced by partial types (or ∞-definability) in simple theories.
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