U-meager sets when the cofinality and the coinitiality of U are uncountable

Abstract

AbtractWe prove that every countably determined set C is U-meager if and only if every internal subset A of C is U-meager, provided that the cofinality and coinitiality of the cut U are both uncountable. As a consequence we prove that for such cuts a countably determined set C which intersects every U-monad in at most countably many points is U-meager. That complements a similar result in [KL]. We also give some partial solutions to some open problems from [KL]. We prove that the set , where H is an infinite integer, cannot be expressed as a countable union of countably determined sets each of which is U-meager for some cut U with min{cf(U), ci(U)} ≥ ω1. Also, every Borel, or countably determined set C which is U-meager for every cut U is a countable union of Borel, or countably determined sets respectively, which are U-nowhere dense for every cut U. Further, the class of Borel U-meager sets for min{cf(U),ci(U)} ≥ ω1 coincides with the least family of sets containing internal U-meager sets and closed with respect to the operation of countable union and intersection. The same is true if the phrase “U-meager sets” is replaced by “U-meager for every cut U.”

Existence of some sparse sets of nonstandard natural numbers

“…PROOF. The idea of the proof is from Proposition 1.5 of [8]. Since cf(C/) is countable and U ^ a -N for any a e * N \ {0}, one can find an increasing sequence (a" : n € co) cofinal in U such that for all n e co, a n+ \ 0 a"N. Suppose the theorem is not true.…”