I was looking through a proof that $\mathbb P^n$ is Hausdorff (take it here as the set of all one dimensional subspaces in $\mathbb{R}^{n+1}$, but it's unclear to me why this isn't nearly trivial.
If I have two points $x,y \in \mathbb{R}^{n+1}$, aren't the images of the disjoint open balls also disjoint open sets in $\mathbb P^n$?
That is, if $x,y \in \mathbb{R}^{n+1}$, and $B_x,B_y$ are disjoint balls, aren't $\pi(B_x)$ and $\pi(B_y)$ disjoint open sets in $\mathbb P^n$? ($\pi : \mathbb{R}^{n+1} \to \mathbb{P}^n$.) Namely, aren't these images the sort of cones in $\mathbb R^{n+1}$ consisting of lines less than $\epsilon$ angle away from the line through $x$ and $0$?
edit: does it work to do the following: let $N$ be sufficiently large
take $L_y$, the line through $y$, and consider the compact subset $\overline{B}_N(x) \cap L_y$. Then, minimize $$\|x-z\|$$ over this subset for $z \in \overline{B}_N(x) \cap L_y$. Repeat for the minimum distance from $y$ to $L_x$. Then use half the smaller of these radii.
Does this work? (i.e. the length of the line segment perpendicular to $L_y$ through $x$)