The pcf-theorem revisited

by Shelah. [Sh:506]
Math Paul Erd\H{o}s, II, 1997
The textrm {pcf} theorem (of the possible cofinality theory) was proved for reduced products prod_{i< kappa} lambda_i/I, where kappa < min_{i< kappa} lambda_i . Here we prove this theorem under weaker assumptions such as wsat(I)< min_{i< kappa} lambda_i, where wsat(I) is the minimal theta such that kappa cannot be delivered to theta sets notin I (or even slightly weaker condition). We also look at the existence of exact upper bounds relative to <_I (<_I-eub) as well as cardinalities of reduced products and the cardinals T_D(lambda) . Finally we apply this to the problem of the depth of ultraproducts (and reduced products) of Boolean algebras

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