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Register a new userOn contractible orbifolds
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Authors
Alexander Lytchak
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We prove that a contractible orbifold is a manifold.
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We study the existence of closed geodesics on compact Riemannian orbifolds, and on noncompact Riemannian manifolds in the presence of a cocompact, isometric group action. We show that every noncontractible Riemannian manifold which admits such an action, and every odd-dimensional, compact Riemannian orbifold has a nontrivial closed geodesic.
We show that on every compact Riemannian 2-orbifold there exist infinitely many closed geodesics of positive length.
We show that the geodesic period spectrum of a Riemannian 2-orbifold all of whose geodesics are closed depends, up to a constant, only on its orbifold topology and compute it. In the manifold case we recover the fact proved by Gromoll, Grove and Pries that all prime geodesics have the same length. In the appendix we partly strengthen our result in terms of conjugacy of contact forms and explain how to deduce rigidity on the real projective plane based on a systolic inequality due to Pu. (We do not use a Lusternik-Schnirelmann type theorem on the existence of at least three simple closed geodesics.)
We characterize Riemannian orbifolds and their coverings in terms of metric geometry. In particular, we show that the metric double of a Riemannian orbifold along the closure of its codimension one stratum is a Riemannian orbifold and that the natural projection is an orbifold covering.
We determine the extent to which the collection of $Gamma$-Euler-Satake characteristics classify closed 2-orbifolds. In particular, we show that the closed, connected, effective, orientable 2-orbifolds are classified by the collection of $Gamma$-Euler-Satake characteristics corresponding to free or free abelian $Gamma$ and are not classified by those corresponding to any finite collection of finitely generated discrete groups. Similarly, we show that such a classification is not possible for non-orientable 2-orbifolds and any collection of $Gamma$, nor for noneffective 2-orbifolds. As a corollary, we generate families of orbifolds with the same $Gamma$-Euler-Satake characteristics in arbitrary dimensions for any finite collection of $Gamma$; this is used to demonstrate that the $Gamma$-Euler-Satake characteristics each constitute new invariants of orbifolds.
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