Cardinalities of DCCC normal spaces with a rank 2-diagonal

Mathematica Bohemica

关键词: cardinality;   

DOI  :  10.21136/MB.2016.0027-15

来源: DOAJ

【 摘 要 】

A topological space $X$ has a rank 2-diagonal if there exists a diagonal sequence on $X$ of rank $2$, that is, there is a countable family $\{\mathcal U_nn\inømega\}$ of open covers of $X$ such that for each $x \in X$, $\{x\}=\bigcap\{{\rm St}^2(x, \mathcal U_n)n \inømega\}$. We say that a space $X$ satisfies the Discrete Countable Chain Condition (DCCC for short) if every discrete family of nonempty open subsets of $X$ is countable. We mainly prove that if $X$ is a DCCC normal space with a rank 2-diagonal, then the cardinality of $X$ is at most $\mathfrak c$. Moreover, we prove that if $X$ is a first countable DCCC normal space and has a $G_\delta$-diagonal, then the cardinality of $X$ is at most $\mathfrak c$.

【 授权许可】

Unknown