olemus
hulkade summa, st hulkadest $A_1, A_2, A_3, \ldots$ moodustatud hulk
$A_1\cup A_2\cup A_3\cup \ldots = \cup_i A_i$ , mis koosneb hulkade $A_1, A_2, A_3, \ldots$
kõigist elementidest, st $\cup_i A_i = \{a\colon\; \exists i\colon a\in A_i \}$
Wiktionary, union:
7. (countable, set theory) the set containing all of the elements of two or more sets

näiteid
https://duckduckgo.com/?t=h_&q=union+of+sets&iax=images&ia=images

ülevaateid
https://en.wikipedia.org/wiki/Union_(set_theory)
https://www.cuemath.com/algebra/union-of-sets/
https://courses.lumenlearning.com/math4libarts/chapter/union-intersection-and-complement/

vt ka
- hulkade ühisosa