Invariant Complex Structures and Kähler Metrics on Principal Bundles

This work investigates the existence and classification of almost complex structures and K\"ahler metrics on principal bundles, with particular emphasis on the homogeneous setting. Using Wang's theory of invariant connections, we give a direct geometric proof of the classification of invariant integrable complex structures established by Biswas and Upmeier. This approach avoids the algebraic machinery of Jordan triple systems and extends the classification from Hermitian symmetric spaces to general symmetric spaces admitting invariant complex structures. We also present a computationally direct proof of Johnson's K\"ahler criterion using the Levi-Civita connection. Finally, we apply these classification results to the reduced frame bundles of the upper half-plane and complex projective spaces, showing that the invariant integrable complex structures in these cases are unique.