Advanced Search

Given a free ultrafilter $p$ on $\mathbb{N}$ and a space $X$, we say that $x\in X$ is the $p$-limit point of a sequence ${\left(x_n\right)}_{n\in \mathbb{N}}$ in $X$ (in symbols, $x=p$-${lim}_{n\to \infty }{x}_n$) if for every neighborhood $V$ of $x$, $\{n\in \mathbb{N}:x_n\in V\}\in p$. By using $p$-limit points from a suitable metric space, we characterize the selective ultrafilters on $\mathbb{N}$ and the $P$-points of ${\mathbb{N}}^{*}=\beta \left(\mathbb{N}\right)\setminus \mathbb{N}$. In this paper, we only consider dynamical systems $(X,f)$, where $X$ is a compact metric space. For a free ultrafilter $p$ on ${\mathbb{N}}^{*}$, the function $f^p:X\to X$ is defined by $f^p\left(x\right)=p$-${lim}_{n\to \infty }{f}^n\left(x\right)$ for each $x\in X$. These functions are not continuous in general. For a...

For a cardinal $\alpha$, we say that a subset $B$ of a space $X$ is $C_{\alpha }$-compact in $X$ if for every continuous function $fX\to {ℝ}^{\alpha }$, $f\left[B\right]$ is a compact subset of ${ℝ}^{\alpha }$. If $B$ is a $C$-compact subset of a space $X$, then $\rho (B,X)$ denotes the degree of $C_{\alpha }$-compactness of $B$ in $X$. A space $X$ is called $\alpha$-pseudocompact if $X$ is $C_{\alpha }$-compact into itself. For each cardinal $\alpha$, we give an example of an $\alpha$-pseudocompact space $X$ such that $X\times X$ is not pseudocompact: this answers a question posed by T. Retta in “Some cardinal generalizations of pseudocompactness”...