Every reference I can find regarding (topological) door spaces gives the following definition almost verbatim:
A door space is one in which every subset is either open or closed. [emphasis mine]
I can think of two interpretations of this definition. Which is the correct one?
- A door space is one in which every subset is open, closed, or both.
- A door space is one in which every nonempty proper subset is either open or closed, but not both.
The former seems plausible because no reference mentions the obvious caveat that the empty set and the entire set are clopen in every topology.
The latter seems plausible based on the fact that every definition uses the word "either" (which, to me, connotes exclusive or) and that physical doors cannot be clopen.