In general topology, one talks about open and closed sets a lot. *A lot*. So it seems a bit silly that there isn’t standard notation for that; it’s sort of like writing “equals” longhand throughout and entire semester of calculus. So I came up with the following simple symbols:

- ( is open in );
- ( is closed in );
- (the interior of A);
- (the closure of A).

They’ve been saving me a lot of time and thought since then, like notation’s supposed to. Witness . The closure symbol, in particular, has ended the ambiguity with , which often denotes the complement of in other contexts. It’s easy to know which is meant if you think about it, but this sort of thing should be run by the cerebellum.