# nLab almost open subspace

A subspace $A$ of a space $X$ is almost open if it is open modulo the $\sigma$-ideal of meagre subspaces?. We also say that $A$ has the Baire property.

Explicitly, $A$ is almost open if there exist an open subspace $G$ and an infinite sequence $N_1, N_2, \ldots$ of nowhere dense subspaces (meaning that their closures have empty interiors) such that

$A \cup \bigcup_i N_i = G \cup \bigcup_i N_i .$

That every subspace of the real line is almost open follows from the axiom of determinacy but contradicts the axiom of choice. In the absence of choice, it is a convenient assumption to make and is one of the axioms of dream mathematics.

Revised on April 12, 2017 01:29:19 by Toby Bartels (108.167.41.14)