The nonexistence of robust codes for subsets of ω₁
Several results are presented concerning the existence or nonexistence, for a subset S of ω₁, of a real r which works as a robust code for S with respect to a given sequence of pairwise disjoint stationary subsets of ω₁, where “robustness” of r as a code may either mean that whenever each is equal to modulo nonstationary changes, or may have the weaker meaning that for every club C ⊆ ω₁. Variants of the above theme are also considered which result when the requirement that S gets exactly...