Billed to the organisers as “a short talk about
a peculiar
topological space that Peter Johnstone, Bob Paré and I used some 30 years ago
to solve a topoidal problem.”
“Peculiar”
It has a discrete, dense, open subset, whose (closed)
complement is a discrete subspace as well, yet ...
“Topoidal
problem”
... the whole space admits a double cover
with no continuous global section.