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Extending compact Hamiltonian $\mathbb{S}^1$-spaces to integrable systems with mild degeneracies in dimension four

Given any compact connected four dimensional symplectic manifold $(M,ω)$ and smooth function $J\colon M\to \mathbb{R}$ which generates an effective $\mathbb{S}^1$-action, we show that there exists a smooth function $H\colon M\to\mathbb{R}$ such that $(M,ω,(J,H))$ is a completely (Liouville) integrable system of a type we call hypersemitoric -- these are systems for which all singularities are non-degenerate, except possibly for a finite number of families of degenerate points of a relatively tame type called parabolic (also sometimes called cuspidal). Such an $(M,ω,J)$ is often referred to as a Hamiltonian $\mathbb{S}^1$-space (classified by Karshon in 1999) and we call any integrable system of the form $(M,ω,(J,H))$ an extension of $(M,ω,J)$. Using this terminology, our main result is that any Hamiltonian $\mathbb{S}^1$-space can be extended to a hypersemitoric integrable system. We also show that there exist Hamiltonian $\mathbb{S}^1$-spaces for which any extension must include at least one degenerate singular point. Parabolic points are among the most common and natural degenerate points, and thus hypersemitoric systems are in this sense the `nicest' class of systems to which all Hamiltonian $\mathbb{S}^1$-spaces can be extended. We also prove several foundational results about these systems, such as the non-existence of loops of hyperbolic-regular points and some properties about their fibers.