I suppose one of the motivations for the “filled ball” symbol is to make it more similar to the symbol for “openness” — I never liked the big typographical difference between the notations for “interior” and “closure”. It does take a little longer to write by hand, but general topology rarely fills pages and pages of calculations, so I find it to be manageable.

Also, I’m much more a “colorful shapes” than “letters” kind of guy: my variables for big, complicated expressions tend to be black/white circles, squares and triangles, or tiny drawings of what the expression represents. )

but the “filled ball” symbol might be a little boring to make it by hand.

for subsets, I find it quite annoying to use \subseteq for usual inclusion, I find it better to use \subset, and in case of proper inclusion, I use \subsetneq.

using A^c for complement and \bar{A} for closure, I suggest using A \subset^\circ X for open sets and A \bar{\subset} X for closed sets.

Cheguei a conclusão de que, além da engenharia e exatas, vc se sairia um ótimo acadêmico de Direito, independente de ser advogado, juiz, promotor, etc, etc…
hehehehe

Como vão as coisas aí em Campinas??

Grande abraço!

–Marco