Georg Cantor at the Dawn of Point-Set Topology - Point-Set Topology

Before Cantor proves his main theorem, he gives several definitions which today we would recognize as belonging to the discipline of point-set topology. Recall Cantor's distinction between values and points above. He first defines a value set to be a finite or infinite set of values (number values). He then defines a point set to be a finite or infinite set of points. Modern mathematics tends to view the term "point-set" as synonymous with "open set." But Cantor's original understanding of point-set is any subset of the real line thought of as being in a one-to-one correspondence with a set of symbols on which you can "do" arithmetic. In fact, it is interesting to note, as G. H. Moore points out [20], that Cantor never used the idea of an open set. Today we define point-set topology in terms of open sets, yet the concept of open set as we know it took dozens of years to develop (again, see [20] for an excellent discussion of the history of open sets).

With a view towards generalizing his theorem, Cantor then defines a cluster point or limit point of a point set $P$ as

a point of the line situated in such a way that each neighborhood of it contains infinitely many points of $P$. (emphasis original)

This is the earliest known published definition of limit point. Cantor’s definition of a neighborhood of a point is "any interval that has the point as its interior" (emphasis original). (Interior point is not defined in this paper, but it would be defined in Cantor's 1879 paper [7].) Now that he has defined limit point, Cantor is able to partition the points of the real line into limit points of $P$ and non-limit points of $P.$ In this way, he defines the first derived set of $P,$ denoted $P^{\prime},$ to be the set of all limit points of $P.$ He may then define the second derived set of $P,$ denoted $P^{\prime\prime},$ as the first derived set of the first derived set $P^{\prime}.$ Continuing in this manner, Cantor defines $P^{(v)},$ the $v$th derived set of $P,$ noting that $P^{(k)}$ may be empty for some $k.$ This allows Cantor to define $P$ to be a point set of the $v$th kind whenever $P^{(v)}$ is finite (and hence $P^{(v+1)}$ is empty).

We construct a point set of the second kind. The reader can then intuit from this example how to construct a point set of the  $v^{th}$ kind for any $v$ (actually writing it down is messy). Let $$A_2=\left\{\frac{1}{n}+\frac{1}{2}: n>2, n\in \Bbb{N}\right\},$$ $$A_3=\left\{\frac{1}{n}+\frac{1}{3}:n>6, n\in \Bbb{N}\right\}, \ldots,$$ $$A_i=\left\{\frac{1}{n}+\frac{1}{i}: n>i(i-1), n\in \Bbb{N}\right\},\dots.$$ This last condition on $n$ ensures that $A_i\subseteq\left[\frac{1}{i},\frac{1}{i-1}\right]$ for $i\geq2.$ Define $P=\bigcup\limits_{i=2}^{\infty}A_i.$ Then $$P'=\left\{\frac{1}{n}: n\geq 2, n\in \Bbb{N}\right\}, P''=\{0\},\,\,{\rm and}\,\,P^{(3)}=\emptyset.$$