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

stabilizer

For a group $G$ acting on $X$ and a point $x\in X$ , the subgroup $\operatorname{Stab}(x)=\{g\in G:g\cdot x=x\}$ of elements leaving $x$ fixed.

Examples

Fixing one vertex of a square under $D_4$ 's action, the stabilizer is $\{e, \text{the reflection through that vertex}\}$ , of order 2.