Cutsets in ${\mathcal P}(X)$

Abstract

For any set $X$, ${\mathcal P}(X)$ denotes the collection of all subsets of $X$, ordered by inclusion. A {\it cutset} in ${\mathcal P}(X)$ is a subset of ${\mathcal P}(X)$ which meets every maximal chain of ${\mathcal P}(X)$. A cutset is non-trivial if it does not contain $X$ or the empty set. Our main result is the following. Theorem 1: Let $X$ be an infinite set of cardinality $\kappa$. Every non-trivial cutset in ${\mathcal P}(X)$ contains a chain of cardinality $\kappa^+$ and an antichain of cardinality $2^{\kappa}$.