Given a Morse-Smale function on an effective orientable orbifold, we construct its Morse-Smale-Witten complex. We show that critical points of a certain type have to be discarded to build a complex properly, and that gradient flows should be counted with suitable weights. Its homology is proven to be isomorphic to the singular homology of the quotient space under the self-indexing assumption. For a global quotient orbifold [M/G], such a complex can be understood as the G-invariant part of the Morse complex of M, where the G-action on generators of the Morse complex has to be defined including orientation spaces of unstable manifolds at the critical points. Alternatively in the case of global quotients, we introduce the notion of weak group actions on Morse-Smale-Witten complexes for non-invariant Morse-Smale functions on M, which give rise to genuine group actions on the level of homology.
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We thank an anonymous referee for suggestions to improve the exposition of the paper. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0000795).
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