Abstract

Let A₁,..., An, An+1 be a finite sequence of algebras of sets given on a set X, $\cup _{k=1}^{n}{\cal A}_{k}\neq \germ{P}(X)$, with more than $\frac{4}{3}n$ pairwise disjoint sets not belonging to An+1. It was shown in [4] and [5] that in this case $\cup _{k=1}^{n+1}{\cal A}_{k}\neq \germ{P}(X)$. Let us consider, instead An+1, a finite sequence of algebras An+1,..., An+l. It turns out that if for each natural i ≤ l there exist no less than $\frac{4}{3}(n+l)-\frac{l}{24}$ pairwise disjoint sets not belonging to An+i, then $\cup _{k=1}^{n+1}{\cal A}_{k}\neq \germ{P}(X)$. But if l ≥ 195 and if for each natural i ≤ l there exist no less than $\frac{4}{3}(n+l)-\frac{l}{15}$ pairwise disjoint sets not belonging to An+i, then $\cup _{k=1}^{n+1}{\cal A}_{k}\neq \germ{P}(X)$. After consideration of finite sequences of algebras, it is natural to consider countable sequences of algebras. We obtained two essentially important theorems on a countable sequence of almost σ-algebras (the concept of almost σ-algebra was introduced in [4])