ON THE ITERATED ω-RULE

Let Γ_n(φ) be a formula of L_PA (PA = Peano Arithmetic) meaning “there is a proof of φ from PA-axioms, in which ω-rule is iterated no more than n times”. We examine relations over pairs of natural numbers of the kind. (n, k) ≦_H (n', k') iff PA + RFN_n' (H_k') ? RFN_n (H_k). Where H denotes one of the hierarchies ∑ or Π and RFN_n(C) is the scheme of the reflection principle for Γ_n restricted to formulas from the class C(Γ_n(φ) implies “φ is true”, for every φ ∈ C). Our main result is that. (n, k) ≦H (n', k') if n ≦ n' and k ≦ max (k', 2n' + 1).     

Fuzzy topology representation for MV‐algebras

Let M be an MV‐algebra and Ω_M be the set of all σ ‐valuations from M into the MV‐unit interval. This paper focuses on the characterization of MV‐algebras using σ ‐valuations of MV‐algebras and proves that a σ ‐complete MV‐algebra is σ ‐regular, which means that a ≤ b if and only if v (a) ≤ v (b) for any v ∈ Ω_M. Then one can introduce in a natural way a fuzzy topology δ on Ω_M. The representation theorem forMV‐algebras is established by means of fuzzy topology. Some properties of fuzzy topology δ and its cut topology U are investigated (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nearly Model Complete Theories

A theory T of a language L is 1-model complete (nearly model complete) iff for every formula ρ of L there is a formula ? (χ) of L which is a ??-formula (a Boolean combination of universal formulas) such that T ? ?x [??θ]. The main results of the paper give characterizations of nearly model complete theories and of 1-model complete theories. As a consequence we obtain that a theory T is nearly model complete iff whenever ?? is a model of T and ???₁??, then T ∪ Δ¹_?? is a complete L(A)-theory, where Δ¹_?? is the 1-diagram of ??. We also point out that our main results extend to (n + l)-model complete and nearly ra-model complete theories for all n > 0.     

Modules with Few Types Over A Serial Ring and Over A Commutative Prüfer Ring

It is proved that the theory of a module M over a countable serial ring has few models iff M is ∑-pure—injective iff the theory of M has few types. It is also proved that the theory of a module M over a countable commutative Prüfer ring has few types if and only if M is ∑-pure—injective.     

On Shattering,Splitting and Reaping Partitions

In this article we investigate the dual-shattering cardinal ?, the dual-splitting cardinal ?? and the dual-reaping cardinal ??, which are dualizations of the well-known cardinals ?? (the shattering cardinal, also known as the distributivity number of P(ω)/fin), s (the splitting number) and ?? (the reaping number). Using some properties of the ideal ?? of nowhere dual-Ramsey sets, which is an ideal over the set of partitions of ω, we show that add(??) = cov(??) = ?. With this result we can show that ? > ω₁ is consistent with ZFC and as a corollary we get the relative consistency of ? > ?? t, where t is the tower number. Concerning ?? we show that cov(M) ? ?? ?? (where M is the ideal of the meager sets). For the dual-reaping cardinal ?? we get p ?? ? ?? ? ?? (where ?? is the pseudo-intersection number) and for a modified dual-reaping number ??′ we get ??′ ? ?? (where ?? is the dominating number). As a consistency result we get ?? < cov(??).     

On Σ1-definable Functions Provably Total in I ∏

We prove the following theorem: Let φ(x) be a formula in the language of the theory PA^? of discretely ordered commutative rings with unit of the form ?yφ′(x,y) with φ′ and let ∈ Δ₀ and let fφ: ? → ? such that f_φ(x) = y iff φ′(x,y) & (?z < y) φ′(x,z). If I ∏ ∈(?x ≥ 0), φ then there exists a natural number K such that I ∏ ? ?y?x(x > y ? ?φ(x) < x^K). Here I ∏₁^? denotes the theory PA^? plus the scheme of induction for formulas φ(x) of the form ?yφ′(x,y) (with φ′) with φ′ ∈ Δ₀.     

Some Lowenheim-Skolem results for admissible sets

For any admissible setA, there is anA-recursive set of sentences of ℒ _A which has a model but noA-finite model.A countable admissible set has the Lowenheim-Skolem property iff it is recurisvely inaccessible and locally countable.     

Strongly compressible modules and semiprime right goldie rings

A module M _R is defined to be strongly compressible (or SC for short) if for every essential submodule N of M, there exists X ? E(M) such that M ? X ? N. We show that a ring R is semiprime right Goldie iff R _Ris SC module iff every right ideal of R is SC module iff every submodule of each progenerator of Mod-R is SC module. As corollaries of this result, we obtain some new module-theoretic characterizations of semiprime Goldie rings, prime (right) Goldie rings and Prüfer rings, etc., etc.,respectively. And the characterization theorem of semiprime Goldie rings of López-Permouth, Rizvi and Yousif by using weakly-injective modules can be regarded as a corollary of our results.     

Relatively Recursively Enumerable Versus Relatively Σ1 in Models of Peano Arithmetic

We show that that every countable model of PA has a conservative extension M with a subset Y such that a certain Σ₁(Y)-formula defines in M a subset which is not r. e. relative to Y.     

Reducing system of parameters and the Cohen-Macaulay property

Let R be a local ring and let (x ₁, …, x _r) be part of a system of parameters of a finitely generated R-module M, where r < dim_R M. We will show that if (y ₁, …, y _r) is part of a reducing system of parameters of M with (y ₁, …, y _r) M = (x ₁, …, x _r) M then (x ₁, …, x _r) is already reducing. Moreover, there is such a part of a reducing system of parameters of M iff for all primes P ε Supp M ∩ V _R(x ₁, …, x _r) with dim_R R/P = dim_R M − r the localization M _P of M at P is an r-dimensional Cohen-Macaulay module over R _P. Furthermore, we will show that M is a Cohen-Macaulay module iff y _d is a non zero divisor on M/(y ₁, …, y _d−1) M, where (y ₁, …, y _d) is a reducing system of parameters of M (d:= dim_R M).     

A perturbative method for the spectral analysis of an acoustic multistratified strip

We consider the acoustic propagator A=−∇·c²∇ in the strip Ω={(x, z)∈ℝ²∣0<z<H} with finite width H>0. The celerity c depends for large ∣x∣ only on the variable z and describes the stratification of Ω: it is assumed to be in L^∞(Ω), bounded from below by c_min>0, such that there exists M>0 with c(x, z)=c₁(z) if x< −M and c(x, z)=c₂(z) if x>M. We look at the propagator A as a ‘perturbation’ of the free propagators A_j in Ω associated to the velocities c_j, j=1, 2, and implement a ‘perturbative’ method, adapting ideas of Majda and Vainberg. The spectrum of A is defined in section 2, a limiting absorption principle is proved in section 3 outside of a countable set Γ(A). The points of Γ(A) can only accumulate at the left of the thresholds of the free propagators. The needed material about A_j, j=1, 2, and some technical estimates for A are given in Appendix. © 1998 B. G. Teubner Stuttgart—John Wiley & Sons, Ltd.     

Automorphisms of Countable Recursively Saturated Models of PA: Open Subgroups and Invariant Cuts

Let M be a countable recursively saturated model of PA and H an open subgroup of G = Aut(M). We prove that I(H) = sup {b ∈ M : (?? ∈ G\H) ?u < b fu = u and J(H) = inf{b ∈ M H} may be invariant, i. e. fixed by all automorphisms of M.     

The List-Chromatic Number of Infinite Graphs Defined on Euclidean Spaces

If D is a countable set of positive reals, 2≤n<ω, let X _n (D) be the graph with the points of R ⁿ as vertices where two vertices are joined iff their distance is in D. We determine the list-chromatic number of X _n (D) as much as possible.     

Inverse limits of hereditarily almost expandable class

In this paper, the hereditarily almost expandability of inverse limits is investigated, with two results obtained. Let X be the inverse limit space of an inverse system （Xα, π^αβ, ∧}. （1） Suppose X is hereditarily κ-metacompact, if each Xα is hereditarily pointwise collectionwise normal （almost θ-expandable, almost discrete θ-expandable）, then so is X; （2） Suppose X is hereditarily κ-σ-metacompact, if each Xα is hereditarily almost σ-expandable （almost discrete σ-expandable = σ-pointwise collectionwise normal）, then so is X.     

A Note on the Embedding of Subspaces of Lp into ℓNr, 0 < r < 1, r ≦ p < 2

In this paper we extend a result by Bourgain-Lindenstrauss-Milman (see [1]). We prove: Let 0 < ? < 1/2, 0< r < 1, r< p < 2. There exists a constant C = C(r,p,?) such that if X is any n-dimensional subspace of L_p(0, l), then there exists Y ? ?^N_r with d(X, Y) ≦ 1 + ?, whenever N > Cn. As an application, we obtain the following partial result: Let 0 < r < 1. There exist constants C = C(r) and C' = C' (r) such that if X is any n-dimensional subspace of L_r(0,1), then there exists Y ? ^N_r with d(X, Y) ≦ C (logn)^l/^r, whenever N ≧ C'n.     

Interstitial and pseudo gaps in models of Peano Arithmetic

In this paper we study the automorphism groups of models of Peano Arithmetic. Kossak, Kotlarski, and Schmerl [9] shows that the stabilizer of an unbounded element a of a countable recursively saturated model of Peano Arithmetic M is a maximal subgroup of Aut(M) if and only if the type of a is selective. We extend this result by showing that if M is a countable arithmetically saturated model of Peano Arithmetic, Ω ? M is a very good interstice, and a ∈ Ω, then the stabilizer of a is a maximal subgroup of Aut(M) if and only if the type of a is selective and rational (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the principles of limiting absorption and limit amplitude for a class of locally perturbed waveguides. Part 1: Time-independent theory

Let Ω be a local perturbation of the n-dimensional domain Ω₀ = Ropf;^{n ? 1} × (0, π). In a previous paper⁸ we have introduced the notion of an admissible standing wave. We shall prove that the principle of limiting absorption holds for the Dirichlet problem of the reduced wave equation in Ω at ω ≥ 0 if Ω does not allow admissible standing waves with frequency ω. From Reference 8, this condition is satisfied for every ω ≥ 0 if Ω ≠ Ω₀, and v · x ′ ≤ 0 on δΩ, where x′ = ( x ₁,…, x_{n ? 1}, 0) and v is the normal unit vector on δΩ pointing into the complement of Ω. In contrast to this, the principle of limiting absorption is violated in the case of the unperturbed domain Ω₀ at the frequencies ω = 1,2,… if n ≤ 3. The second part of our investigation, which will appear in a subsequent paper, is devoted to the principle of limit amplitude.     

The spectrum problem III: Universal theories

We solve the classification problem and essentially the spectrum problem for universal theories (see [6] for discussion of the meaning of this). We first solve it forT such that ifM ₁,M ₂ elementarily extendM ₀ and are independent over it, then overM ₀ ∪M ₁ there is a prime model. This generalizes [2]. This was subsequently used and generalized for countable first order theories. (This will appear in [5].) But note that there the theory is countable and in the case of structure the model is prime over a non-forking tree of models; here the model is generated by the union (and theT not necessarily countable). The universality is used in Theorey.If T is stable and complete then either (A)for every M ₁<M (l=0, 1, 2)models of T, if M ₀ ⊆M ₁,M ₂, {M ₁,M ₂}is independent over M ₀ (i.e. tp(M ₁,M ₂)is finitely satisfiable in M ₀),then the submodel of M which M ₁ ⋃M ₂ generates is an elementary submodel of M, or (B)there is an unstable theory extending the universal part of T (we can replace universal by Σ₂ and slightly more). Conclusion. For any universalT:Either (a) for every modelM ofT there is a treeI with ≦ω levels and submodelsN _η (η ∈I) of power ≦2^|T| (by [5], just ≦|T|) such that (i)M is generated by ∪_ηεl N _η, (ii)η <v⇒⇒N _η, (iii) ifv is an immediate successor ofη then tp(N _v, ⋃{{N _p:ρ ∈I,v≨ϱ}) is finitely satisfiable inN _η (note that asking this just for quantifier-free formulas is enough).Or (b) for every cardinalλ>|T|, there are 2^γ non-isomorphic models for powerλ. Dedicated to Professor Abraham Robinson The author would like to thank John Baldwin for the interesting talks in September 1980 which led to §3 of this work, Rami Grossberg for various corrections, and the BSF and NSF for their partial support. This paper was originally intended to appear in the Proceedings of the Model Theory Year at the Institute for Advanced Studies, The Hebrew University of Jerusalem, September 1980 — August 1981, published in Isr. J. Math., Vol. 49, Nos. 1–3, 1984.     

More on regular and decomposable ultrafilters in ZFC

We prove, in ZFC alone, some new results on regularity and decomposability of ultrafilters; among them: (a) If m ≥ 1 and the ultrafilter D is (_m(λ⁺ⁿ), _m(λ⁺ⁿ))‐regular, then D is κ ‐decomposable for some κ with λ ≤ κ ≤ 2^λ (Theorem 4.3(a^')). (b) If λ is a strong limit cardinal and D is (_m(λ⁺ⁿ), _m(λ⁺ⁿ))‐regular, then either D is (cf λ, cf λ)‐regular or there are arbitrarily large κ < λ for which D is κ ‐decomposable (Theorem 4.3(b)). (c) Suppose that λ is singular, λ < κ, cf κ ≠ cf λ and D is (λ⁺, κ)‐regular. Then: (i) D is either (cf λ, cf λ)‐regular, or (λ^', κ)‐regular for some λ^' < λ (Theorem 2.2). (ii) If κ is regular, then D is either (λ, κ)‐regular, or (ω, κ^')‐regular for every κ^' < κ (Corollary 6.4). (iii) If either (1) λ is a strong limit cardinal and λ^<λ < 2^κ, or (2) λ^<λ < κ, then D is either λ ‐decomposable, or (λ^', κ)‐regular for some λ^' < λ (Theorem 6.5). (d) If λ is singular, D is (μ, cf λ)‐regular and there are arbitrarily large ν < λ for which D is ν ‐decomposable, then D is κ ‐decomposable for some κ with λ ≤ κ ≤ λ^<μ (Theorem 5.1; actually, our result is stronger and involves a covering number). (e) D × D^' is (λ, μ)‐regular if and only if there is a ν such that D is (ν, μ)‐regular and D^' is (λ, ν^')‐regular for all ν^～ < ν (Proposition 7.1). We also list some problems, and furnish applications to topological spaces and to extended logics (Corollar‐ies 4.6 and 4.8) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of g∞,σ‐integral operators

The application of the general tensor norms theory of Defant and Floret to the ideal of (p, σ)‐absolutely continuous operators of Matter, 0 < σ < 1, 1 ≤ p < ∞ leads to the study of g_p′,σ‐nuclear and g_p′,σ‐integral operators. Characterizations of such operators has been obtained previously in the case p > 1. In this paper we characterize the g_∞,σ‐nuclear and g_∞,σ‐integral operators by the existence of factorizations of some special kinds. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)