Saturday, 9 December 2006

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

For any antiholomorphic Diffeomorphism fcolonStoSfcolonStoS we get a canonical identification fstarbarK=K,fstarbarK=K, KK and barKbarK being the canonical and anticanonical bundle of the Riemann surface. A holomorphic structure on a complex vectorbundle EE is the same as an complex operator
DcolonGamma(E)toGamma(barKE)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, fEfE has a natural complex structure (it's just i).
Therefore one gets an anti-holomorphic structure barDcolonGamma(fstarE)toGamma(KfE)barDcolonGamma(fstarE)toGamma(KfE) satisfying the antiholomorphic Cauchy Riemann equation.
But the complex conjugate bundle barEbarE also has a anti-holomorphic structure, since overlinebarKE=KbarE.overlinebarKE=KbarE. Therefore, fbarEfbarE has a natural holomorphic structure.



These two holomorphic structures are not isomorphic in general:
In the case of a line bundle L=EL=E of degree 00 one might see this as follows: every holomorphic structure DD gives rise to an unique unitary flat connection nablanabla such that
D=1/2(nabla+inabla).D=1/2(nabla+inabla). Then the anti-holomorphic structure on barLbarL is given by 1/2(nablainabla)1/2(nablainabla) and, the unitary flat connection corresponding to the holomorphic structure on fLfL is the connection fnabla.fnabla. But this connection is not gauge equivalent to nablanabla in general: For example, on a square torus with f given by zmapstobarzzmapstobarz the connection d+cidxd+cidx is not gauge equivalent to d+cidyd+cidy for cinRsetminus2piZ.cinRsetminus2piZ.

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