Lebesque measure zero subsets of the real line and an infinite game

Extract

Let denote the ideal of Lebesgue measure zero subsets of the real line. Then add() denotes the minimal cardinality of a subset of whose union is not an element of . In [1] Bartoszynski gave an elegant combinatorial characterization of add(), namely: add() is the least cardinal number κ for which the following assertion fails:

For every family of at mostκ functions from ω to ω there is a function F from ω to the finite subsets of ω such that:

1. For each m, F(m) has at most m + 1 elements, and

2. for each f inthere are only finitely many m such that f(m) is not an element of F(m).

The symbol A(κ) will denote the assertion above about κ. In the course of his proof, Bartoszynski also shows that the cardinality restriction in 1 is not sharp. Indeed, let (R_m: m < ω) be any sequence of integers such that for each m R_m, ≤ R_m+1, and such that lim_m→∞R_m = ∞. Then the truth of the assertion A(κ) is preserved if in 1 we say instead that

1′. For each m, F(m) has at most R_m elements.

We shall use this observation later on. We now define three more statements, denoted B(κ), C(κ) and D(κ), about cardinal number κ.