Relationship between Chern and Levi-Civita Connections on Kahler Manifolds

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SUMMARY

The discussion centers on the relationship between the Levi-Civita connection on the real tangent space and the Chern connection on the holomorphic tangent space of a complex manifold with a Hermitian metric. It is established that these connections coincide if and only if the metric is Kähler. The isomorphism between the real tangent space and the holomorphic tangent space is clarified through specific transformations of partial derivatives. The participant seeks to understand the implications of these connections and the conditions under which the Levi-Civita connection induces the Chern connection.

PREREQUISITES
  • Understanding of complex manifolds and Hermitian metrics
  • Familiarity with the concepts of Levi-Civita and Chern connections
  • Knowledge of tangent spaces and their isomorphisms
  • Basic grasp of Christoffel symbols and their role in connections
NEXT STEPS
  • Study the properties of Kähler metrics in complex geometry
  • Explore the derivation and applications of the Chern connection
  • Investigate the conditions under which the Levi-Civita connection induces the Chern connection
  • Review literature on the relationship between real and holomorphic tangent bundles
USEFUL FOR

Mathematicians, particularly those specializing in differential geometry, complex geometry, and anyone researching the interplay between Levi-Civita and Chern connections on Kähler manifolds.

cogito²
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So I'm trying to understand the statement: On a complex manifold with a hermitian metric the Levi-Civita connection on the real tangent space and the Chern connection on the holomorphic tangent space coincide iff the metric is Kahler.

I basically understand the meaning of this statement, but I'm becoming incredibly confused by the details. The isomorphism between the real tangent space and the holomorphic tangent space is basically just (as far as I've understood it):

\frac{\partial}{\partial x_i} \mapsto \frac{\partial}{\partial z_i} = \frac{1}{2}(\frac{\partial}{\partial x_i} - i\frac{\partial}{\partial y_i})

\frac{\partial}{\partial y_i} \mapsto i\frac{\partial}{\partial z_i} = \frac{1}{2}(\frac{\partial}{\partial x_i} + i\frac{\partial}{\partial y_i})

with the holomorphic space viewed as sitting inside the complexified tangent space.

So now what I wonder is the following. What exactly does

\nabla_\frac{\partial}{\partial x_i} \frac{\partial}{\partial x_j}

correspond to in terms of the Chern connection? Is it simply this?

\nabla_\frac{\partial}{\partial z_i} \frac{\partial}{\partial z_j}

What I mean is that

\nabla_\frac{\partial}{\partial x_i} \frac{\partial}{\partial x_j} = A_{i j}^k \frac{\partial}{\partial x_k} + B_{i j}^k \frac{\partial}{\partial y_k}

so does this mean then that the following is true?

\nabla_\frac{\partial}{\partial z_i} \frac{\partial}{\partial z_j} = (A_{i j}^k + iB_{i j}^k)\frac{\partial}{\partial z_k}

If that we're the case, then it would seem like

\nabla_\frac{\partial}{\partial x_i} f\frac{\partial}{\partial x_j} \neq \nabla_\frac{\partial}{\partial z_i} f\frac{\partial}{\partial z_j}

because

\frac{\partial f}{\partial x_i} \neq \frac{\partial f}{\partial z_i}

So I guess I'm just confused exactly what is meant in the books talking about this. I'm having trouble truly understanding the proofs I've found because I can't understand exactly what identifications are being made. If anyone can shed light on my problems or knows of a book that does this pretty explicitly I would be very thankful.
 
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I have thought about this some more and have become convinced that the following identification is correct:

Let X be a complex manifold and (TX,J) be the real tangent space with canonical complex structure. Then (TX,J) is isomorphic to the holomorphic tangent bundle (T_X, i) where the isomorphism connects the action of J to that of i. Then say you have a connection \nabla on TX such that \nabla J = 0. Then you can put a real connection on T_X which will preserve the action of i. I.e.:

\nabla : TX \times TX \to TX
\nabla_\frac{\partial}{\partial x_i} \frac{\partial}{\partial x_j}

corresponds to:

\nabla : TX \times T_X \to T_X
\nabla_\frac{\partial}{\partial x_i} \frac{\partial}{\partial z_j}

Then because \nabla J = 0 you get

\nabla_\frac{\partial}{\partial x_i} i\frac{\partial}{\partial z_j} = i\nabla_\frac{\partial}{\partial x_i} \frac{\partial}{\partial z_j}

Then after that you can just extend

\nabla : TX \times T_X \to T_X

to

\nabla : TX \otimes_\mathbb{R} \mathbb{C} \times T_X \to T_X

linearly. Then this will be a complex connection. Now you can ask under which conditions the induced complex connection on T_X will end up being a holomorphic connection. I've written up some fairly unilluminating conditions on the Christoffel symbols for this to be the case.

Now if you have a real metric g on TX and a hermitian metric h on T_X (and you'd probably like h to be induced by g), you can ask whether under this process the Levi-Civita connection will induce the Chern connection.

I'm now fairly certain I am understanding this correctly, however if any of you see that I'm still doing something wrong, I'd appreciate the input! Also I'm still interested in seeing any source that does this clearly.
 

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