Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

index of a subgroup

Written $[G:H]$ , the number of distinct left cosets of a subgroup $H$ in $G$ -- equivalently $|G|/|H|$ for a finite group, by Lagrange's theorem.

Examples

$[\mathbb{Z}_6 : \{0,3\}] = 3$ .