Space S000050

Rationals extended by a focal point

Also known as: Non-Hausdorff cone over the rationals, Open extension of the rationals

Let $X = \mathbb{Q} \cup \{\infty\}$, where $\mathbb{Q}$ has the usual topology on Rational numbers and is an open subset of $X$, and where the only neighborhood of $\infty$ is $X$. See Has a point with a unique neighborhood.

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