olemus
rühma alamhulk, mis on ise rühm sama binaartehte mõttes

Wiktionary:
2. (group theory) a subset H of a group G that is itself a group and has the same binary operation as G

ülevaateid
https://en.wikipedia.org/wiki/Subgroup

http://www.maths.lth.se/matematiklth/personal/ssilvest/AlgebraVT2011/Lecture09Silvestrov.pdf