Title: A generalization of a theorem of Hurewicz for quasi-Polish spaces,
Abstract: We identify four countable topological spaces $S_2$, $S_1$, $S_D$, and $S_0$
which serve as canonical examples of topological spaces which fail to be
quasi-Polish. These four spaces respectively correspond to the $T_2$, $T_1$,
$T_D$, and $T_0$-separation axioms. $S_2$ is the space of rationals, $S_1$ is
the natural numbers with the cofinite topology, $S_D$ is an infinite chain
without a top element, and $S_0$ is the set of finite sequences of natural
numbers with the lower topology induced by the prefix ordering. Our main result
is a generalization of Hurewicz's theorem showing that a co-analytic subset of
a quasi-Polish space is either quasi-Polish or else contains a countable
$\Pi^0_2$-subset homeomorphic to one of these four spaces.