We study the intersection theory of complex Lagrangian subvarieties inside holomorphic symplectic manifolds. In particular, we study their behaviour under Mukai flops and give a rigorous proof of the Plücker type formula for Legendre dual subvarieties written down by the second author before. Then we apply the formula to study projective dual varieties in projective spaces.
Intersection theory of complex Lagrangians in hyper-Kähler manifoldsIn this section, we recall some standard facts about the intersection theory of complex Lagrangian subvarieties inside holomorphic symplectic manifolds.2.1. Some basic notions from hyper-Kähler geometry. Let (M, g) be a hyper-Kähler manifold with complex structures I, J, K which satisfy the Hamilton relationThen the twistor family of complex structures on M can be expressed as S 2 = {aI + bJ + cK | a 2 + b 2 + c 2 = 1 and a, b, c ∈ R} Fixing a complex structure J, (M, g, J) is a Calabi-Yau manifold which corresponds to the embedding Sp(n) ⊆ SU (2n). We denote the Kähler form by ω J . The parallel form Ω J = ω I − iω K defines a holomorphic symplectic structure on (M, J). And the J-holomorphic volume form for the Calabi-Yau structure on (M, g, J) is just the top exterior power of Ω J .