> Who invented the idea of an open set? Was it Weierstrass? If you have
> a reference as well, that would be nice.
<q>
Cantor in 1872 introduced the concept of the first derived set, or set of
limit points, of a set. He also defined closed subsets of the real line as
subsets containing their first derived set. Cantor also introduced the idea
of an open set another fundamental concept in point set topology.
</q>
http://turnbull.dcs.st-and.ac.uk/history/HistTopics/Topology_in_mathematics.html
APH