olemus
topoloogilise ruumi lahtise alamhulga täiend

Wiktionary:
(topology) a set whose complement is open

ülevaateid
https://mathworld.wolfram.com/ClosedSet.html

https://en.wikipedia.org/wiki/Closed_set

http://planning.cs.uiuc.edu/node126.html

http://www.pitt.edu/~luca/ECON2001/lecture_04.pdf