A direct proof of a theorem of Jech and Shelah on PCF algebras

Abstract

By using an argument based on the structure of the locally compact scattered spaces, we prove in a direct way the following result shown by Jech and Shelah: there is a family {Bα : α < ω1} of subsets of ω1 such that the following conditions are satisfied: (a) max Bα - α, (b) if α ∈ Bβ then Bα ⊆ Bβ, (c) if δ ≤ α and δ is a limit ordinal then Bα ∩ δ is not in the ideal generated by the sets Bβ, β < α, and by the bounded subsets of δ, (d) there is a partition {An : n ∈ ω} of ω1 such that for every α and every n, Bα ∩An is finite.