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On inverse gamma-systems and the number of L_{infty,lambda}-equivalent, non-isomorphic models for lambda singular

Saharon Shelah·Pauli Väisänen
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Abstract

Suppose lambda is a singular cardinal of uncountable cofinality kappa. For a model M of cardinality lambda, let No(M) denote the number of isomorphism types of models N of cardinality lambda which are L_{infty lambda}-equivalent to M. In [Sh:189] inverse kappa-systems A of abelian groups and their certain kind of quotient limits Gr(A)/Fact(A) were considered. It was proved that for every cardinal mu there exists an inverse kappa-system A such that A consists of abelian groups having cardinality at most mu^kappa and card(Gr(A)/Fact(A))= mu. In [Sh:228] a strict connection between inverse kappa-systems and possible values of No was proved. In this paper we show: for every nonzero mu <= lambda^kappa there is an inverse kappa-system A of abelian groups having cardinality < lambda such that card(Gr(A)/Fact(A))= mu (under the assumptions 2^kappa < lambda and theta^{< kappa}< lambda for all theta < lambda when mu > lambda), with the obvious new consequence concerning the possible value of No. Specifically, the case No(M)= lambda is possible when theta^kappa < lambda for every theta < lambda.

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Saharon Shelah, Pauli Väisänen. 1998-07-15. On inverse gamma-systems and the number of L_{infty,lambda}-equivalent, non-isomorphic models for lambda singular. https://arxiv.org/abs/math/9807181

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