In this paper, we study smooth, semi-free actions on closed, smooth, simply connected manifolds, such that the orbit space is a smoothable manifold. We show that the only simply connected -manifolds admitting a smooth, semi-free circle action with fixed-point components of codimension are connected sums of -bundles over . Furthermore, the Betti numbers of the -manifolds and of the quotient -manifolds are related by a simple formula involving the number of fixed-point components. We also investigate semi-free actions on simply connected -manifolds with quotient a -manifold and show, in particular, that there are strong restrictions on the topology of the -manifold.
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John Harvey, Martin Kerin, Krishnan Shankar, Semi-Free Actions with Manifold Orbit Spaces. Doc. Math. 25 (2020), pp. 2085–2114DOI 10.4171/DM/794