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Abstract

We investigate the structure of FN bases (Frechet-Nikodym bases) without assuming the Continuum Hypothesis (CH), refining results of Siu-Ah Ng concerning definability via flatness and nonforking. In particular, we examine the dependence of Theorem 3.4 and Corollary 3.5 of Ng's 1991 paper on CH, which guarantees the existence of nonforking primary heirs under specific ultrafilter conditions. We demonstrate that these properties can often be recovered in ZFC by analyzing the behavior of countably incomplete, good, and regular ultrafilters. By isolating model-theoretic conditions sufficient to replace CH, we establish that ultrapowers of flat FN bases satisfying Property B remain definable and nonforking. Connections are drawn to canonical bases, coheirs, and stability-theoretic ranks in superstable theories. Examples and counterexamples clarify the precise role of ultrafilter properties and reveal that the combinatorial essence of Ng's theory can be preserved within a purely ZFC framework.