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On the number of -equivalent non-isomorphic models
Authors:Saharon Shelah   Pauli Vä  isä  nen
Affiliation:Institute of Mathematics, The Hebrew University, Jerusalem, Israel and Rutgers University, Hill Ctr-Busch, New Brunswick, New Jersey 08903 ; Department of Mathematics, P.O. Box 4, 00014 University of Helsinki, Finland
Abstract:

We prove that if $operatorname{ZF}$ is consistent then $operatorname{ZFC} + operatorname{GCH}$ is consistent with the following statement: There is for every $k < omega$ a model of cardinality $aleph_1$ which is $L_{infty{omega_{1}}}$-equivalent to exactly $k$non-isomorphic models of cardinality $aleph_1$. In order to get this result we introduce ladder systems and colourings different from the ``standard' counterparts, and prove the following purely combinatorial result: For each prime number $p$ and positive integer $m$ it is consistent with $operatorname{ZFC} + operatorname{GHC}$ that there is a ``good' ladder system having exactly $p^m$ pairwise nonequivalent colourings.

Keywords:Number of models   ladder system   uniformization   infinitary logic   iterated forcing
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