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We show that every orientable 3--manifold is a classifying space
BΓ where Γ is a groupoid of germs of
homeomorphisms of R. This follows by showing that every
orientable 3--manifold M admits a codimension one foliation
F such that the holonomy cover of every leaf is
contractible. The F we construct can be taken to be C¹
but not C². The existence of such an F answers
positively a question posed by Tsuboi [Classifying spaces for groupoid
structures, notes from minicourse at PUC, Rio de Janeiro (2001)], but
leaves open the question of whether M = BΓ for some
C∞ groupoid Γ.
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