In part, if n is even then d ∗ = − ∗ d∗. Similarly, Fact 5.1.2. ∂ ∗ = − ∗ ∂∗ and ∂ ∗ = − ∗ ∂∗ are formal adjoint of ∂ and ∂ with respect to the L 2 metric in A k C . Proof: (∂α, β) = ∫ X ∂α ∧ ∗β = − ∫ X (−1)|α| α ∧ ∂ ∗ β = − ∫ X (−1)|α| α ∧ ∗ ∗ −1 ∂ ∗ β = (α, ∂ ∗ β). More generally, if (E, h) is a Hermitian vector bundle then there exists a C-anti linear isomorphism of vector bundles given by h : Ω 0,q X ⊗ E = Ω0,q (E) −→ (Ω 0,q ⊗ E) ∨ ∼ = Ω n,n−q ⊗ E ∨ , where ∆ 2n X = Ωn,n = RdVol(x). So it gives am antilinear isomorphism ∗ E : Ω 0,q (E) ∼ −→ Ω n,n−q (E ∨ ) = K X ⊗ Ω 0,n−q (E ∨ ) called the Hodge Star, where K X = Ω n,0 = ∆ n C T X ∨ bundle of a complex manifold X. (holomorphic line bundle) is called the canonical Fact 5.1.3. ∂ ∗ X = (−1) q ∗ −1 E ◦∂ K X ⊗E ∨ : A0,q (E) −→ A 0,q−1 (E) is the formal adjoint of ∂ E . Fact 5.1.4. (d ∗ ) 2 = (∂ ∗ E) 2 = (∂ ∗ ) 2 = 0. Definition 5.1.2. Let (X, g) be a Riemannian manifold, ∆ := dd ∗ + d ∗ d = (d + d ∗ ) 2 Definition 5.1.3. Let (X, h) be a Hermitian manifold, ∆ ∂ := ∂∂ ∗ + ∂ ∗ ∂ = (∂ + ∂ ∗ ) 2 , ∆ ∂ := ∂∂ ∗ + ∂ ∗ ∂ = (∂ + ∂ ∗ ) 2 . If further E −→ X is a holomorphic vector bundle with a Hermit metric, we write ∆ E for ∆ ∂E = (∂ E +∂ ∗ E) 2 . From construction, 〈α, ∆ d α〉 = ||dα|| 2 + ||d ∗ α|| 2 and analogously for the Hermitian case. Corollary 5.1.1. Ker(∆ d ) = Ker(d) ∩ Ker(d ∗ ). Definition 5.1.4. An element of Ker(∆ d ) is called harmonic, i.e., it is killed by d and d ∗ . 64
Theorem 5.1.1 (Main Theorem of the <strong>Course</strong>). Let { ( ⊕k Ω (F, φ) = ( k X , ∆ d) ) ⊕q Ω 0,q (E), ∆ ∂E for the Riemannian case, for the Hermitian case. where φ : F −→ F is an automorphism, and F = ⊕ k Ω k X or F = ⊕ qΩ 0,q (E). 65