olemus
hulk H koos selle hulga mingi alamhulkade hulgaga, millesse kuuluvad alamhulgad on lahtised hulgad, kusjuures
(i) nii H ise kui ka tühi hulk on lahtised
(ii) mistahes lahtiste hulkade ühend on samuti lahtine hulk
(iii) mistahes kahe lahtise hulga ühisosa on lahtine hulk

Merriam-Webster:
: a set with a collection of subsets satisfying the conditions that both the empty set and the set itself belong to the collection, the union of any number of the subsets is also an element of the collection, and the intersection of any finite number of the subsets is an element of the collection
= a space which has an associated family of subsets that constitute a topology. The relationships between members of the space are mathematically analogous to those between points in ordinary two- and three-dimensional space.

näiteid
reaalarvude hulga alamhulk on lahtine, kui temas on iga reaalarvu r korral vahemik (a,b), milles a < r < b

ülevaateid
https://christopherolah.files.wordpress.com/2011/04/spaces1.png
https://en.wikipedia.org/wiki/Topological_space
http://mathworld.wolfram.com/TopologicalSpace.html
https://www.encyclopediaofmath.org/index.php/Topological_space
https://www.maths.tcd.ie/~pete/ma2223/chapter2.pdf

vt ka
- topoloogia