Spaces of small cellularity have nowhere constant continuous images of small weight

Abstract

We call a continuous map $f:X\to Y$ nowhere constant if it is not constant on any non-empty open subset of its domain $X$. Clearly, this is equivalent with the assumption that every fiber $f^{-1}(y)$ of $f$ is nowhere dense in $X$. We call the continuous map $f:X\to Y$ pseudo-open if for each nowhere dense $Z\subset Y$ its inverse image $f^{-1}(Z)$ is nowhere dense in $X$. Clearly, if $Y$ is crowded, i.e. has no isolated points, then $f$ is nowhere constant. The aim of this paper is to study the following, admittedly imprecise, question: How "small" nowhere constant, resp. pseudo-open continuous images can "large" spaces have? Our main results yield the following two precise answers to this question, explaining also our title. Both of them involve the cardinal function $\hat{c}(X)$, the "hat version" of cellularity, which is defined as the smallest cardinal $\kappa$ such that there is no $\kappa$-sized disjoint family of open sets in $X$. Thus, for instance, $\hat{c}(X)={\omega }_1$ means that $X$ is CCC. THEOREM A. Any crowded Tychonov space $X$ has a crowded Tychonov nowhere constant continuous image $Y$ of weight $w(Y)\le \hat{c}(X)$. Moreover, in this statement $\le$ may be replaced with $<$ iff there are no $\hat{c}(X)$-Suslin lines (or trees). THEOREM B. Any crowded Tychonov space $X$ has a crowded Tychonov pseudo-open continuous image $Y$ of weight $w(Y)\le 2^{<\hat{c}(X)}$. If Martin's axiom holds then there is a CCC crowded Tychonov space $X$ such that for any crowded Hausdorff pseudo-open continuous image $Y$ of $X$ we have $w(Y)\ge \mathfrak{c}(=2^{<{\omega }_1})$.

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