A flop formula for Donaldson-Thomas invariants

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Let X and X' be nonsingular projective 3-folds related by a flop of a disjoint union of (-2)-curves. We prove a flop formula relating the Donaldson-Thomas invariants of X to those of X', which implies some simple relations among BPS state counts. As an application, we show that if X satisfies the GW/DT correspondence for primary insertions and descendants of the point class, then so does X'. We also propose a conjectural flop formula for general flops.