For more details about index sets and partitions please visit
https://matherialien.jimdofree.com/
and read the pdf under "Correction / addition to lesson 6 (and 15)"
00:00 What is a left (right) coset?
01:07 Abelian example
02:59 Nonabelian example
04:39 A useful Lemma (and corollary) about cosets
08:12 Abelian example revisited
09:32 Proposition 18
11:45 Example of a group partitioned into cosets
14:26 The (left) index of a subgroup
15:17 Examples of indices