Fintushel–Stern's Montgomery–Yang problem

A pseudo-free differentiable circle action on the $5$-dimensional sphere is an action whose non-free orbits are isolated orbit types. Fintushel–Stern's Montgomery–Yang problem. Every pseudo-free differentiable circle action on the $5$-dimensional sphere has at most three non-free orbits. This is the original topological formulation motivating the algebraic version considered in the paper; its resolution status is not specified in the supplied text.