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1. Torus actions, maximality, and non-negative curvature

   https://doi.org/10.1515/crelle-2021-0035  

   Escher, Christine; Searle, Catherine (August 2021, Journal für die reine und angewandte Mathematik (Crelles Journal))

   Abstract Let ℳ 0 n {\mathcal{M}_{0}^{n}} be the class of closed, simply connected, non-negatively curved Riemannian n -manifolds admitting an isometric, effective, isotropy-maximal torus action. We prove that if M ∈ ℳ 0 n {M\in\mathcal{M}_{0}^{n}} , then M is equivariantly diffeomorphic to the free, linear quotient by a torus of a product of spheres of dimensions greater than or equal to 3. As a special case, we then prove the Maximal Symmetry Rank Conjecture for all M ∈ ℳ 0 n {M\in\mathcal{M}_{0}^{n}} . Finally, we showthe Maximal Symmetry Rank Conjecture for simply connected, non-negatively curved manifolds holds for dimensions less than or equal to 9 without additional assumptions on the torus action. 

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2. On 3-manifolds that are boundaries of exotic 4-manifolds

   https://doi.org/10.1090/tran/8586  

   Etnyre, John; Min, Hyunki; Mukherjee, Anubhav (October 2022, Transactions of the American Mathematical Society)

   We give several criteria on a closed, oriented 3-manifold that will imply that it is the boundary of a (simply connected) 4-manifold that admits infinitely many distinct smooth structures. We also show that any weakly fillable contact 3-manifold, or contact 3-manifold with non-vanishing Heegaard Floer invariant, is the boundary of a simply connected 4-manifold that admits infinitely many distinct smooth structures each of which supports a symplectic structure with concave boundary, that is there are infinitely many exotic caps for any such contact manifold. 

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3. The Novikov conjecture, the group of volume preserving diffeomorphisms and Hilbert-Hadamard spaces

   Gong, Sherry; Wu, Jianchao; Yu, Guoliang (August 2024, Geometric and functional analysis)

   We prove that the Novikov conjecture holds for any discrete group admitting an isometric and metrically proper action on an admissible Hilbert-Hadamard space. Admissible Hilbert-Hadamard spaces are a class of (possibly infinite-dimensional) non-positively curved metric spaces that contain dense sequences of closed convex subsets isometric to Riemannian manifolds. Examples of admissible Hilbert-Hadamard spaces include Hilbert spaces, certain simply connected and non-positively curved Riemannian-Hilbertian manifolds and infinite-dimensional symmetric spaces. Thus our main theorem can be considered as an infinite-dimensional analogue of Kasparov’s theorem on the Novikov conjecture for groups acting properly and isometrically on complete, simply connected and non-positively curved manifolds. As a consequence, we show that the Novikov conjecture holds for geometrically discrete subgroups of the group of volume preserving diffeomorphisms of a closed smooth manifold. This result is inspired by Connes’ theorem that the Novikov conjecture holds for higher signatures associated to the Gelfand-Fuchs classes of groups of diffeormorphisms. 

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4. Almost isotropy-maximal manifolds of non-negative curvature

   https://doi.org/10.1090/9100  

   Dong, Zheting; Escher, Christine; Searle, Catherine (May 2024, Transactions of the American Mathematical Society)

   We extend the equivariant classification results of Escher and Searle for closed, simply connected, Riemannian $n$-manifolds with non-negative sectional curvature admitting isometric isotropy-maximal torus actions to the class of such manifolds admitting isometric strictly almost isotropy-maximal torus actions. In particular, we prove that any such manifold is equivariantly diffeomorphic to the free, linear quotient by a torus of a product of spheres of dimensions greater than or equal to three. 

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5. Special cubulation of strict hyperbolization

   https://doi.org/10.1007/s00222-024-01241-9  

   Lafont, Jean-François; Ruffoni, Lorenzo (June 2024, Inventiones mathematicae)

   Abstract We prove that the Gromov hyperbolic groups obtained by the strict hyperbolization procedure of Charney and Davis are virtually compact special, hence linear and residually finite. Our strategy consists in constructing an action of a hyperbolized group on a certain dual$$\operatorname {CAT}(0)$$ $CAT\left(0\right)$cubical complex. As a result, all the common applications of strict hyperbolization are shown to provide manifolds with virtually compact special fundamental group. In particular, we obtain examples of closed negatively curved Riemannian manifolds whose fundamental groups are linear and virtually algebraically fiber. 

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