Past exam of the options I course of the University of Oxford / 2006 / A / A1

Write the group action as $(\omega ,g)\mapsto \omega g$. A right group action satisfies $\omega 1=\omega$ and $(\omega g)h=\omega (gh)$. Its orbit and stabilizer areThe identity belongs to $H$. If $g,h\in H$, then $\omega gh=(\omega g)h=\omega$, so $gh\in H$. Also $\omega g=\omega$ implies $\omega =(\omega g)g^{-1}=\omega g^{-1}$, so $g^{-1}\in H$. Thus $H$ is a subgroup.

For the orbit-stabilizer theorem, the map $G\to \mathrm{Orb}(\omega )$, $g\mapsto \omega g$, is onto, andIts fibers are precisely the right cosets $Hh$, each containing $|H|$ elements. Counting these disjoint fibers givesFor the symmetric group $S_3$, the six subgroups areThe group action $H\mapsto g^{-1}Hg$ has four orbits: each of $\{1\}$, $A_3$, and $S_3$ is fixed, while the three order-two subgroups form one orbit. Here an orbit is a collection of subgroups, rather than a collection of individual permutations.