F.E.J. Linton • Wesleyan Univ. Math/CS Emeritus

These are the slides for a presentation at the New York City Category Seminar on October 19, 2009, about a peculiar topological space that Peter
Johnstone, Bob Paré and I used some 30 years ago to solve a
topoidal problem.

"Short"? —
Could actually be presented in well under a half-hour.

"Peculiar"? — this space 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.

Seemed like the sort of mix of areas NYC Category Theory Seminar participants would appreciate. And the pictures may be a pleasant diversion, too :-) .