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24
Modal Languages And Bounded Fragments Of Predicate Logic
, 1996
"... Model Theory. These are nonempty families I of partial isomorphisms between models M and N , closed under taking restrictions to smaller domains, and satisfying the usual BackandForth properties for extension with objects on either side  restricted to apply only to partial isomorphisms of size ..."
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Cited by 210 (12 self)
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Model Theory. These are nonempty families I of partial isomorphisms between models M and N , closed under taking restrictions to smaller domains, and satisfying the usual BackandForth properties for extension with objects on either side  restricted to apply only to partial isomorphisms of size at most k . 'Invariance for kpartial isomorphism' means having the same truth value at tuples of objects in any two models that are connected by a partial isomorphism in such a set. The precise sense of this is spelt out in the following proof. 21 Proof (Outline.) kvariable formulas are preserved under partial isomorphism, by a simple induction. More precisely, one proves, for any assignment A and any partial isomorphism IÎI which is defined on the Avalues for all variables x 1 , ..., x k , that M, A = f iff N , IoA = f . The crucial step in the induction is the quantifier case. Quantified variables are irrelevant to the assignment, so that the relevant partial isomorphism can be res...
Back and Forth Between Modal Logic and Classical Logic
, 1994
"... Model Theory. That is, we have a nonempty family I of partial isomorphisms between two models M and N, which is closed under taking restrictions to smaller domains, and where the standard BackandForth properties are now restricted to apply only to partial isomorphisms of size at most k. Proof. ..."
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Cited by 30 (3 self)
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Model Theory. That is, we have a nonempty family I of partial isomorphisms between two models M and N, which is closed under taking restrictions to smaller domains, and where the standard BackandForth properties are now restricted to apply only to partial isomorphisms of size at most k. Proof. (A complete argument is in [16].) An outline is reproduced here, for convenience. First, kvariable formulas are preserved under partial isomorphism, by a simple induction. More precisely, one proves, for any assignment A and any partial isomorphism I 2 I which is defined on the Avalues for all variables x 1 ; : : : ; x k , that M;A j= OE iff N; I ffi A j= OE: The crucial step in the induction is the quantifier case. Quantified variables are irrelevant to the assignment, so that the relevant partial isomorphism can be restricted to size at most k \Gamma 1, whence a matching choice for the witness can be made on the opposite side. This proves "only if". Next, "if" has a proof analogous to...
The Expressive Power of Finitely Many Generalized Quantifiers
 Information and Computation
, 1995
"... We consider extensions of first order logic (FO) and fixed point logic (FP) by means of generalized quantifiers in the sense of Lindstrom. We show that adding a finite set of such quantifiers to FP fails to capture PTIME, even over a fixed signature. This strengthens results in [10] and [15]. We als ..."
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Cited by 24 (5 self)
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We consider extensions of first order logic (FO) and fixed point logic (FP) by means of generalized quantifiers in the sense of Lindstrom. We show that adding a finite set of such quantifiers to FP fails to capture PTIME, even over a fixed signature. This strengthens results in [10] and [15]. We also prove a stronger version of this result for PSPACE, which enables us to establish a weak version of a conjecture formulated in [16]. These results are obtained by defining a notion of element type for bounded variable logics with finitely many generalized quantifiers. Using these, we characterize the classes of finite structures over which the infinitary logic L ! 1! extended by a finite set of generalized quantifiers Q is no more expressive than first order logic extended by the quantifiers in Q . 1 Introduction Computational complexity measures the complexity of a problem in terms of the resources, such as time, space, or hardware, required to solve the problem relative to a given ma...
Inflationary Fixed Points in Modal Logic
, 2002
"... We consider an extension of modal logic with an operator for constructing... ..."
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Cited by 18 (8 self)
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We consider an extension of modal logic with an operator for constructing...
First Order Logic, Fixed Point Logic and Linear Order
, 1995
"... The Ordered conjecture of Kolaitis and Vardi asks whether fixedpoint logic differs from firstorder logic on every infinite class of finite ordered structures. In this paper, we develop the tool of bounded variable element types, and illustrate its application to this and the original conjectures o ..."
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Cited by 16 (0 self)
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The Ordered conjecture of Kolaitis and Vardi asks whether fixedpoint logic differs from firstorder logic on every infinite class of finite ordered structures. In this paper, we develop the tool of bounded variable element types, and illustrate its application to this and the original conjectures of McColm, which arose from the study of inductive definability and infinitary logic on proficient classes of finite structures (those admitting an unbounded induction). In particular, for a class of finite structures, we introduce a compactness notion which yields a new proof of a ramified version of McColm's second conjecture. Furthermore, we show a connection between a modeltheoretic preservation property and the Ordered Conjecture, allowing us to prove it for classes of strings (colored orderings). We also elaborate on complexitytheoretic implications of this line of research.
How to Define a Linear Order on Finite Models
, 1997
"... We carry out a systematic investigation of the definability of linear order on classes of finite rigid structures. We obtain upper and lower bounds for the expressibility of linear order in various logics that have been studied extensively in finite model theory, such as least fixpoint logic LFP, pa ..."
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Cited by 14 (1 self)
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We carry out a systematic investigation of the definability of linear order on classes of finite rigid structures. We obtain upper and lower bounds for the expressibility of linear order in various logics that have been studied extensively in finite model theory, such as least fixpoint logic LFP, partial fixpoint logic PFP, infinitary logic L ! 1! with a finite number of variables, as well as the closures of these logics under implicit definitions. Moreover, we show that the upper and lower bounds established here can not be made substantially tighter, unless outstanding conjectures in complexity theory are resolved at the same time. Research of L. Hella was partially supported by a grant from the University of Helsinki. y Research of Ph. Kolaitis was partially supported by a 1993 John Simon Guggenheim Fellowship and by NSF Grants No. CCR9108631, CCR9307758, and INT9024681 z Research of K. Luosto was partially supported by a grant from the Emil Aaltonen Foundation. 1 Intro...
Interpolation, Preservation, and Pebble Games
 Journal of Symbolic Logic
, 1996
"... Preservation and interpolation results are obtained for L1! and sublogics L ` L1! such that equivalence in L can be characterized by suitable backandforth conditions on sets of partial isomorphisms. 1 Introduction In the heyday of infinitary logic in the 1960's and 70's, most attention was focuse ..."
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Cited by 12 (5 self)
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Preservation and interpolation results are obtained for L1! and sublogics L ` L1! such that equivalence in L can be characterized by suitable backandforth conditions on sets of partial isomorphisms. 1 Introduction In the heyday of infinitary logic in the 1960's and 70's, most attention was focused on L!1! and its fragments (see e.g. Keisler [19]), since countable formulas seemed best behaved. The past decade has seen a renewed interest in L1! and its finite variable fragments L (k) 1! (for 2 k ! !) and the modal fragment L \Pi 1! (see e.g. Ebbinghaus and Flum [17] on the former and Barwise and Moss [9] on the latter), due to various connections with topics in computer science. These logics form a hierarchy of increasingly powerful logics L \Pi 1! ae L (2) 1! ae L (3) 1! ae : : : ae L (k) 1! ae : : : ae L1! ; with each of these inclusions being proper. Moreover, there is a useful and elegant algebraic characterization of equivalence in L in each of these logics L, from b...
Finite Variable Logics In Descriptive Complexity Theory
 Bulletin of Symbolic Logic
, 1998
"... this article. ..."