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Saharon Shelah

Publications and source records attributed to Saharon Shelah.

At least 523 records · Page 29Linked to original sources
arXiv2001

Sweet & Sour and other flavours of ccc forcing notions

The present paper has three themes. First, we continue the investigations started in Judah, Roslanowski and Shelah \math.LO/9310224 and Roslanowski and Shelah math.LO/9807172, math.LO/9703222, and we investigate the method of norms on possibilities in the context of ccc forcing notions, getting a number of constructions of nicely definable ccc forcings. The second theme of the paper is a part of the general program ``how special are random and Cohen forcing notions (or: the respective ideals)''. Shelah math.LO/9303208 shows that the two forcing notions may occupy special positions in the realm of nicely definable forcing notions. In this realm we may classify forcing notions using the methods of Shelah [Sh:630] (math.LO/9712283), [Sh:669] and, for example, declare that very Souslin (or generally omega-nw-nep) ccc forcing notions are really nice. Both the Cohen forcing notion and the random forcing notion and their FS iterations (and nice subforcings) are all ccc omega-nw-nep, and Problem 4.24 of math.LO/9906113 asked if we have more examples. It occurs that our method relatively easily results in very Souslin ccc forcing notions. The third theme is Sweet & Sour, and it is related to one of the most striking differences between the random and the Cohen forcing notions that appears when we consider the respective regularity properties of projective sets: the Lebesgue measurability of Sigma^1_3 sets implies aleph_1 is inaccessible in L, while one can construct (in ZFC) a forcing notion P which forces ``projective subsets of R have the Baire property'' (see Shelah [Sh:176]).

math.LO↗
arXiv2001

Functional Equations for Lexicographic Products

We generalize the main result of math.RA/9608214 concerning the convex embeddings of a chain Gamma in a lexicographic power Delta^Gamma. For a fixed nonempty chain Delta, we derive necessary and sufficient conditions for the existence of nonempty solutions Gamma to each of the lexicographic functional equations (Delta^Gamma)^{<=0} simeq Gamma, (Delta^Gamma) simeq Gamma, and (Delta^Gamma)^{<0} simeq Gamma.

math.LO↗
arXiv2001

Weak Diamond

Under some cardinal arithmetic assumptions, we prove that every stationary subset of lambda of a right cofinality has the weak diamond. This is a strong negation of uniformization. We then deal with a weaker version of the weak diamond- colouring restrictions. We then deal with semi- saturated (normal) filters.

math.LO↗
arXiv2001

Some ugly aleph_1-free abelian groups

Given an aleph_1-free abelian group G we characterize the class C_G of all torsion abelian groups T satisfying Ext(G,T)=0 assuming the continuum hypothesis CH. Moreover, in Godel's constructable universe we prove that this characterizes C_G for arbitrary torsion-free abelian G. It follows that there exist some ugly aleph_1-free abelian groups.

math.LO↗
arXiv2001

On embedding models of arithmetic of cardinality aleph_1 into reduced powers

In the early 1970's S.Tennenbaum proved that all countable models of PA^- + forall_1-Th(N) are embeddable into the reduced product N^omega/F, where F is the cofinite filter. In this paper we show that if M is a model of PA^- + forall_1-Th(N), and |M|= aleph_1, then M is embeddable into N^omega/D, where D is any regular filter on omega.

math.LO↗
arXiv2001

On regular reduced products

Assume -> . Assume M is a model of a first order theory T of cardinality at most lambda^+ in a vocabulary L(T) of cardinality <= lambda . Let N be a model with the same vocabulary. Let Delta be a set of first order formulas in L(T) and let D be a regular filter on lambda. Then M is Delta-embeddable into the reduced power N^lambda/D, provided that every Delta-existential formula true in M is true also in N. We obtain the following corollary: for M as above and D a regular ultrafilter over lambda, M^lambda/D is lambda^{++}-universal. Our second result is as follows: For i -> holds. We show that then the second player has a winning strategy in the Ehrenfeucht-Fraisse game of length lambda^+ on prod_i M_i/D and prod_i N_i/D. This yields the following corollary: Assume GCH and lambda regular). For L, M_i and N_i as above, if D is a regular filter on lambda, then prod_i M_i/D cong prod_i N_i/D .

math.LO↗
arXiv2001

Model Companions of T_σfor stable T

Let T be a complete first order theory in a countable relational language L . We assume relation symbols have been added to make each formula equivalent to a predicate. Adjoin a new unary function symbol sigma to obtain the language L_sigma; T_sigma is obtained by adding axioms asserting that sigma is an L-automorphism. We provide necessary and sufficient conditions for T_sigma to have a model companion when T is stable. Namely, we introduce a new condition: T_sigma admits obstructions, and show that T_sigma has a model companion iff and only if T_sigma does not admit obstructions. This condition is weakening of the finite cover property: if a stable theory T has the finite cover property then T_sigma admits obstructions.

math.LO↗
arXiv2001

On needed reals

Following Blass, we call a real a ``needed'' for a binary relation R on the reals if in every R-adequate set we find an element from which a is Turing computable. We show that every real needed for Cof(N) is hyperarithmetic. Replacing ``R-adequate'' by ``R-adequate with minimal cardinality'' we get related notion of being ``weakly needed''. We show that is is consistent that the two notions do not coincide for the reaping relation. (They coincide in many models.) We show that not all hyperarithmetical reals are needed for the reaping relation. This answers some questions asked by Blass at the Oberwolfach conference in December 1999.

math.LO↗
arXiv2001

Definability of initial segments

We consider implicit definability of the standard part {0,1,...} in nonstandard models of Peano arithmetic (PA), and we ask whether there is a model of PA in which the standard part is implicitly definable. In section 1, we define a certain class of formulas, and show that in any model of PA the standard part is not implicitly defined by using such formulas. In section 2 we construct a model of PA in which the standard part is implicitly defined. To construct such a model, first we assume a set theoretic hypothesis diamondsuit_{S_lambda^{lambda^+}}, which is an assertion of the existence of a very general set. Then we shall eliminate the hypothesis using absoluteness for the existence of a model having a tree structure with a certain property.

math.LO↗
arXiv2001

On the existence of rigid aleph_1-free abelian groups of cardinality aleph_1

An abelian group is said to be aleph_1-free if all its countable subgroups are free. Our main result is: If R is a ring with R^+ free and |R|<lambda <= 2^{aleph_0}, then there exists an aleph_1-free abelian group G of cardinality lambda with End(G)=R . A corollary to this theorem is: Indecomposable aleph_1-free abelian groups of cardinality aleph_1 do exist.

math.LO↗
arXiv2001

Coding with ladders a well-ordering of the reals

Any model of ZFC + GCH has a generic extension (made with a poset of size aleph_2) in which the following hold: MA + 2^{aleph_0}= aleph_2+ there exists a Delta^2_1-well ordering of the reals. The proof consists in iterating posets designed to change at will the guessing properties of ladder systems on omega_1. Therefore, the study of such ladders is a main concern of this article.

math.LO↗
arXiv2001

A tall space with small bottom

We introduce a general method of constructing locally compact scattered spaces from certain families of sets and then, with the help of this method, we prove that if kappa^{<kappa}=kappa then there is such a space of height kappa^+ with only kappa many isolated points. This implies that there is a locally compact scattered space of height omega_2 with omega_1 isolated points in ZFC, solving an old problem of the first author.

math.LO↗
arXiv2001

Evasion and prediction IV: Fragments of constant prediction

Say that a function pi:n^{ n (henceforth called a predictor) k-constantly predicts a real x in n^omega if for almost all intervals I of length k, there is i in I such that x(i)=pi (x restriction i). We study the k-constant prediction number v_n^const(k), that is, the size of the least family of predictors needed to k --constantly predict all reals, for different values of n and k, and investigate their relationship.

math.LO↗
arXiv2001

On the lattice of cotorsion theories

We discuss the lattice of cotorsion theories for abelian groups. First we show that the sublattice of the well-studied rational cotorsion theories can be identified with the well-known lattice of types. Using a recently developed method for making Ext vanish we also prove that any power set together with the ordinary set inclusion (and thus any poset) can be embedded into the lattice of all cotorsion theories.

math.LO↗
arXiv2001

You Can Enter Cantor's Paradise

This paper is based on the talk given by the author after he received the International Bolyai Prize in Mathematics (on November 4, 2000 in Budapest, Hungary).

math.LO↗