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Ivancevic_Applied-Diff-Geom

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756 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introductionhas H 2 (X ′ , C) = 0. (The necessary C a are explicitly described later.) Soif we restrict to X ′ , the cohomology class of ω vanishes and λ exists. λmay however have poles on the C a , perhaps with residues, which we callRes Ca (λ). 30 If λ does have residues, then dλ contains delta functions, andif one works on X instead of X ′ , one really has not (4.249) butω = dλ − 2πi ∑ aRes Ca (λ) · [C a ] (4.250)where [C a ] (which represents the cohomology class known as the Poincarédual of C a ) is a delta function supported on C a .In cohomology, (4.250) simply means[ω] = −2πi ∑ aRes Ca (λ) · [C a ]. (4.251)Thus, if we pick the C a so that the [C a ] are a basis of H 2 (X, C), thenthe residues Res Ca (λ) are simply the coefficients of the expansion of [ω]in terms of the [C a ]. To find the residues we need not actually find λ; itsuffices to understand the cohomology class of ω by any method that maybe available.For instance, if X were compact, we could proceed as follows. Firstcompute the intersection matrixM ab = #(C a · C b )(that is, the number of intersection points of C a and C b , after perhapsperturbing the C a so that they intersect generically). This is an invertiblematrix. Second, calculate the periods∫c a = ω.C aThen[ω] = ∑ a,bc a M −1ab [C b].Comparing to (4.246), we get0.Res Ca (λ) = − 12πi∑bM −1ab c b.30 The residues of λ along C a are constants, since dRes Ca (λ) = Res Ca dλ = Res Ca ω =

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