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Closed almost Kähler 4-manifolds of constant non-negative Hermitian holomorphic sectional curvature are Kähler. (English) Zbl 1484.53084

Let \(M\) be a closed almost Kähler 4-manifold. The authors study the conditions such that \(M\) is Kähler. It is shown that among these conditions the assumptions for holomorphic sectional curvature play an important role. Assuming that \(M\) is of pointwise constant Hermitian holomorphic sectional curvature \(k\geq 0\), the authors prove that \(M\) is Kähler-Einstein, holomorphically isometric to \(\mathbb{C}P\) with the Fubini-Study metric (for \(k>0\)) and a complex torus or a hyperelliptic curve with the Ricci-flat Kähler metric (for \(k=0\)). Furthermore, assuming that \(k<0\) and the Ricci tensor is \(J\)-invariant they prove that \(M\) is Kähler-Einstein, holomorphically isometric to a compact quotient of the complex hyperbolic ball \(\mathbb{B}^4\) with the Bergman metric.
To prove previously mentioned results the authors use algebraic methods at the beginning, upon that they exploit the index theorems for the signature and the Euler characteristic using Chern-Weil theory, as well as some results from Seiber-Witten theory, and formulas for the Bach tensor.
At the end some open problems concerning non-compact manifolds, Lie groups, counterexamples to Schur’s theorem, the twistor space approach are formulated.

MSC:

53C25 Special Riemannian manifolds (Einstein, Sasakian, etc.)
53C15 General geometric structures on manifolds (almost complex, almost product structures, etc.)

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