Saturday, 9 December 2006

dg.differential geometry - Definition of a complex structure on a vector bundle

For any antiholomorphic Diffeomorphism fcolonStoS we get a canonical identification fstarbarK=K, K and barK being the canonical and anticanonical bundle of the Riemann surface. A holomorphic structure on a complex vectorbundle E is the same as an complex operator
DcolonGamma(E)toGamma(barKE) satisfying the (Cauchy Riemann) Leibnitz rule. (holomorphic sections are exactly the one in the kernel of D). (For higher dimensions this is not true.)



Now, fE has a natural complex structure (it's just i).
Therefore one gets an anti-holomorphic structure barDcolonGamma(fstarE)toGamma(KfE) satisfying the antiholomorphic Cauchy Riemann equation.
But the complex conjugate bundle barE also has a anti-holomorphic structure, since overlinebarKE=KbarE. Therefore, fbarE has a natural holomorphic structure.



These two holomorphic structures are not isomorphic in general:
In the case of a line bundle L=E of degree 0 one might see this as follows: every holomorphic structure D gives rise to an unique unitary flat connection nabla such that
D=1/2(nabla+inabla). Then the anti-holomorphic structure on barL is given by 1/2(nablainabla) and, the unitary flat connection corresponding to the holomorphic structure on fL is the connection fnabla. But this connection is not gauge equivalent to nabla in general: For example, on a square torus with f given by zmapstobarz the connection d+cidx is not gauge equivalent to d+cidy for cinRsetminus2piZ.

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