Groupoids, Branched Manifolds and Multisections

Cieliebak, Mundet i Riera and Salamon recently formulated a definition of branched submanifold of Euclidean space in connection with their discussion of multivalued sections and the Euler class. This note proposes an intrinsic definition of a weighted branched manifold Z that is obtained from the usual definition of oriented orbifold groupoid by relaxing the properness condition and adding a weighting. We show that if Z is compact, finite dimensional and oriented, then it carries a fundamental class [Z]. Adapting a construction of Liu and Tian, we also show that the fundamental class [X] of any oriented orbifold X may be represented by a map Z → X, where the branched manifold Z is unique up to a natural equivalence relation. This gives further insight into the structure of the virtual moduli cycle in the new polyfold theory recently constructed by Hofer, Wysocki and Zehnder.