| الوصف: |
In this paper, we investigate the poset $\mathbf{OF}(X)$ of free open filters on a given space $X$. In particular, we characterize spaces for which $\mathbf{OF}(X)$ is a lattice. For each $n\in\mathbb{N}$ we construct a scattered space $X$ such that $\mathbf{OF}(X)$ is order isomorphic to the $n$-element chain, which implies the affirmative answer to two questions of Mooney. Assuming CH we construct a scattered space $X$ such that $\mathbf{OF}(X)$ is order isomorphic to $(\omega+1,\geq)$. To prove the latter facts we introduce and investigate a new stratification of ultrafilters which depends on scattered subspaces of $\beta(\kappa)$. Assuming the existence of $n$ measurable cardinals, for every $m_0,\ldots,m_{n}\in\mathbb N$ we construct a space $X$ such that $\mathbf{OF}(X)$ is order isomorphic to $\prod_{i=0}^nm_i$. Also, we show that the existence of a metric space possessing a free $\omega_1$-complete closed, $G_\delta$, $F_{\sigma}$ or Borel ultrafilter is equivalent to the existence of a measurable cardinal. |
