A bicategory of reduced orbifolds from the point of view of differential geometry - I

We describe a bicategory (Red Orb) of reduced orbifolds in the framework of differential geometry (i.e. without any explicit reference to notions of Lie groupoids or differentiable stacks, but only using orbifold atlases, local lifts and changes of charts). In order to construct such a bicategory, we first define a 2-category (Red Atl) whose objects are reduced orbifold atlases (on paracompact, second countable, Hausdorff topological spaces). The definition of morphisms is obtained as a slight modification of a definition by A. Pohl, while the definition of 2-morphisms and compositions of them is new in this setup. Using the bicalculus of fractions described by D. Pronk, we are able to construct from such a 2-category the bicategory (Red Orb). We prove that it is equivalent to the bicategory of reduced orbifolds described in terms of proper, effective, étale Lie groupoids by D. Pronk and I. Moerdijk and to the 2-category of reduced orbifolds described by several authors in the past in terms of a suitable class of differentiable Deligne-Mumford stacks.