Recently, the following tag synonyms were created by a MathOveflow moderator: abstract-algebra $\to$ ra.rings-and-algebras, geometry $\to$ mg.metric-geometry and discrete-mathematics $\to$ co.combinatorics. You can see the new synonyms in the list of tag synonyms (Wayback Machine). The three tags...

Let $X = \bigcup_{n\geq 0} L_n$ where $L_n = [0, 1)\times\{1/i\}$ for $i > 0$ and $L_0 = (0, 1)\times \{0\}$. Define the topology on $X$ as follows: each point $(x, 1/i)$ for $x\in (0, 1)$ and $i > 0$ is isolated, neighbourhoods of $(0, 1/i)$ are subsets $A$ of $L_i$ containing $(0, 1/i)$ with $L...