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Displaying 181 – 200 of 729

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On Distributive Fixed-Point Expressions

Helmut Seidl, Damian Niwiński (2010)

RAIRO - Theoretical Informatics and Applications

For every fixed-point expression e of alternation-depth r, we construct a new fixed-point expression e' of alternation-depth 2 and size 𝒪 ( r · | e | ) . Expression e' is equivalent to e whenever operators are distributive and the underlying complete lattice has a co-continuous least upper bound. We alternation-depth but also w.r.t. the increase in size of the resulting expression.

On divisibility in definable groups

Margarita Otero (2009)

Fundamenta Mathematicae

Let ℳ be an o-minimal expansion of a real closed field. It is known that a definably connected abelian group is divisible. We show that a definably compact definably connected group is divisible.

On dot-depth two

F. Blanchet-Sadri (1990)

RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications

On embedding models of arithmetic of cardinality ℵ₁ into reduced powers

Juliette Kennedy, Saharon Shelah (2003)

Fundamenta Mathematicae

In the early 1970’s S. Tennenbaum proved that all countable models of PA₁¯ + ∀₁ -Th(ℕ) are embeddable into the reduced product ω / , where ℱ is the cofinite filter. In this paper we show that if M is a model of PA¯ + ∀₁ - Th(ℕ), and |M| = ℵ₁, then M is embeddable into ω / D , where D is any regular filter on ω.

On equivalence relations second order definable over H(κ)

Saharon Shelah, Pauli Vaisanen (2002)

Fundamenta Mathematicae

Let κ be an uncountable regular cardinal. Call an equivalence relation on functions from κ into 2 second order definable over H(κ) if there exists a second order sentence ϕ and a parameter P ⊆ H(κ) such that functions f and g from κ into 2 are equivalent iff the structure ⟨ H(κ), ∈, P, f, g ⟩ satisfies ϕ. The possible numbers of equivalence classes of second order definable equivalence relations include all the nonzero cardinals at most κ⁺. Additionally, the possibilities are closed under unions...

On Existentially First-Order Definable Languages and Their Relation to NP

Bernd Borchert, Dietrich Kuske, Frank Stephan (2010)

RAIRO - Theoretical Informatics and Applications

Under the assumption that the Polynomial-Time Hierarchy does not collapse we show for a regular language L: the unbalanced polynomial-time leaf language class determined by L equals  iff L is existentially but not quantifierfree definable in FO[<, min, max, +1, −1]. Furthermore, no such class lies properly between NP and co-1-NP or NP⊕co-NP. The proofs rely on a result of Pin and Weil characterizing the automata of existentially first-order definable languages.

On expansions of weakly o-minimal non-valuational structures by convex predicates

Roman Wencel (2009)

Fundamenta Mathematicae

We prove that if ℳ = (M,≤,+,...) is a weakly o-minimal non-valuational structure expanding an ordered group (M,≤,+), then its expansion by a family of "non-valuational" unary predicates remains non-valuational. The paper is based on the author's earlier work on strong cell decomposition for weakly o-minimal non-valuational expansions of ordered groups.

Currently displaying 181 – 200 of 729