Ordinary topology on infinite subsets ($\tau$)

The ordinary topology on $[\mathbb{N}{]}^{\omega }$ has basic sets $[s]=\{X:s⊏X\}$ for finite $s$. It is the product topology on increasing enumerations, and also the subspace topology on infinite-subset indicator functions in $\{0,1{\}}^{\mathbb{N}}$.

In the ordinary topology on infinite subsets, the family $D$ of cofinite sets is a countable set, a dense and a meagre set. Countability follows by indexing its members by their finite complements. Each singleton is closed with empty interior, while every basic cylinder with finite initial segment $s$ contains $s\cup \{n:n>\max s\}$. Consequently $D$ is not a nowhere dense set. This example uses the ordinary cylinder topology, rather than the Ramsey cone topology.

The family $D=\{\{a_1<a_2<\cdots \}:a_{n+1}>2a_n\text{ for all }n\}$ is ordinarily closed and nowhere dense. A failed gap is witnessed by a finite prefix, and any cylinder can be refined by adjoining consecutive large integers that fail the gap. Nevertheless $D$ intersects every infinite-subset cone in continuum many sets: a binary tree chooses two sufficiently large next points inside its reservoir at each step.

A closed nowhere dense family can meet every cone $[M{]}^{\omega }$ in continuum many sets, as the gap-doubling closed family of infinite subsets does. Well-order the cones and choose two fresh candidates in each, permanently reserving one of each colour. The red family meets every cone and its complement contains every reserved blue candidate, so it is not Ramsey. As a subset of the closed nowhere dense family, it is still nowhere dense and has the Baire property in the ordinary infinite-subset topology. This recursion requires no regularity assumption on the continuum cardinal.

A family $Y\subseteq [\mathbb{N}{]}^{\omega }$ is $\tau$-Baire when $Y△O$ is meagre for some ordinarily open family $O$. This is weaker than the corresponding regularity in the star topology. A meagre set meeting every Ramsey cone in both colours has the ordinary Baire property while failing to be a Ramsey set of infinite subsets.