Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2015/iii/paper-18/4/a/solution

The Chern connection of a Hermitian metric on a holomorphic vector bundle is the unique connection on a vector bundle compatible with that metric and whose part is the bundle's Dolbeault operator . Fix a holomorphic local frame and column coefficients for sections. Write the metric as , conjugate-linear in the first argument, and write the connection as . The condition forces to have type . Metric compatibility requires
where the dagger conjugates the differential-form coefficients as well as transposing the matrix. Taking the part gives . Its conjugate-transpose supplies the metric equation because is Hermitian. This proves uniqueness and local existence.
Under a holomorphic change of frame , the metric matrix becomes . The local formula for the Chern connection on a vector bundle then gives
This is precisely the transformation rule for a connection on a vector bundle, so the local connections glue and establish global existence. No Kähler hypothesis is needed for this part.
Extend to vector-bundle-valued differential forms by the graded Leibniz rule. The curvature form of a connection is the tensorial square , acting by exterior multiplication. In the chosen frame,
Indeed by differentiating . It follows that has type ; the gauge change is . Thus it is a global smooth two-form with values in , namely an element of .

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