Double-coset criterion for a one-point extension
= Double-coset criterion for a one-point extension
If $x$ interchanges the new point and $\alpha$, put $H=G_\alpha$. Then $\langle G,x\rangle$ is a one-point extension exactly when $x^2\in H$, $xHx^{-1}=H$, and $xgx\in GxG$ for every $g\notin H$. These conditions make $G\cup GxG$ multiplication-closed, hence a group in the finite setting, with new-point stabilizer $G$. Necessity comes from the two double cosets in the extended two-transitive action.