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
https://webspace.maths.qmul.ac.uk/m.jerrum/MTH6126/note4.pdf
https://www.math.nagoya-u.ac.jp/~richard/teaching/f2022/SML_Adam_1.pdf