Can anyone tell me the origin &/or original applications of Smirnov's Deleted Sequence topology? ([This is #64 in Steen & Seebach's *Counterexamples in Topology*](https://i.stack.imgur.com/tCCAD.png).) Thanks.



Smirnov's Deleted Sequence topology
The example appears already in Alexandroff and Hopf's Topologie (1935) in Chapter 1, section 1, number 4, Example 1.
It served as an example of a Hausdorff non-regular space.


