Surfaces moving by powers of Gauss curvature

We prove that strictly convex surfaces moving by $K^{\alpha \mathrm{/}2}$ become spherical as they contract to points, provided $\alpha$ lies in the range $[1,2]$. In the process we provide a natural candidate for a curvature pinching quantity for surfaces moving by arbitrary functions of curvature, by finding a quantity conserved by the reaction terms in the evolution of curvature.