Lusin spaces as images of locally compact Polish spaces

Abstract

A Lusin space is a Hausdorff space being the image of a Polish space under a continuous bijection. Such spaces have multiple applications, in particular, as state spaces of various stochastic systems. In this work, we consider the spaces obtained as the images of a noncompact and locally compact Polish space $(X, \mathcal{T})$, which we call $c$-Lusin. The main result is the statement that a $c$-Lusin space $Y=f(X)$, can be written as $Z\cup Y_1$, where $Z$ is a locally compact Polish space whereas $Y_1$ is $c$-Lusin. At the same time, $Y_1$ is the set of the discontinuity points of $f^{-1}$ which is a closed subset of $Y$. Moreover, $Y_1$ is nowhere dense if (and only if) $Y$ is a Baire space. By the same arguments, $Y_1$ can also be decomposed as $Z_1 \cup Y_2$ with the properties as above. In the case where $f$ can be extended to a continuous map $f:X\cup \{\infty\} \to Y$, and thus $Y_1$ is a singleton, we construct a metric on $X$ such that the corresponding metric space is compact and homeomorphic to the $c$-Lusin space $(f(X), \mathcal{T}')$.