S. Shelah et P. Vaisanen, On inverse gamma-systems and the number of L-infinity lambda-equivalent, non-isomorphic models for lambda singular, J SYMB LOG, 65(1), 2000, pp. 272-284
Suppose lambda is a singular cardinal of uncountable cofinality kappa. Tor
a model M of cardinality lambda. let No (M) denote the number of isomorphis
m types of models N of cardinality lambda which are L-infinity lambda-equiv
alent to M. In [7] Shelah considered inverse kappa-systems A of abelian gro
ups and their certain kind of quotient limits Gr(A)/ Fact(A). In particular
Shelah proved in [7. Fact 3.10] that for every cardinal mu there exists an
inverse kappa-system A such that A consists of abelian groups having cardi
nality at most mu(kappa) and card (Gr(A)/Fact(A)) = mu. Later in [8, Theore
m 3.3] Shelah showed a strict connection between inverse K-systems and poss
ible values of No (under the assumption that 0(kappa) < lambda for every 0
< lambda): if A is an inverse kappa-system of abelian groups having cardina
lity < lambda, then there is a model M such that card (M) = lambda and No(M
) = card(Gr(A)/Fact(A)). The following was an immediate consequence (when 0
(kappa) < lambda for every 0 < lambda): for every nonzero mu < lambda or mu
= lambda(kappa) there is a model M-mu of cardinality lambda with No(M-mu)
= mu. In this paper we show: for every nonzero mu less than or equal to lam
bda(kappa) there is an inverse kappa-system A of abelian groups having card
inality < lambda such that card(Gr(A)/Fact(A)) = mu(under the assumptions 2
(kappa) < lambda and 0 (<)kappa <lambda for all 0 < lambda when mu > lambda
), with the obvious new consequence concerning the possible value of No. Sp
ecifically. the case No(M) = lambda is possible when 0(kappa) < lambda for
every 0 < lambda.