# Kähler structure on open complex manifold

Is it always possible to put a compatible Kähler metric on an open complex manifold (non-compact, without boundary, say of finite topological type)?

Edit 1: Michael Albanese pointed out a nice counterexample (namely the punctured Hopf surface) when this is impossible for topological reasons. Question: Would it be possible to have counter-examples of the following unpleasant type: The underlying open manifold has a Kähler metric, but for a given complex structure we cannot find a compatible Kähler metric?

Edit 2: Misha pointed out there is a Kahler metric on punctured $S^3 \times S^1$. Thinking more about this, I cannot resist to ask one final variation. Question Does there exist an open complex manifold for which there is no Kahler metric on the underlying smooth manifold?

• A Hopf surface with a point removed does not admit a Kähler metric, see this question. – Michael Albanese May 12 '17 at 11:10

## 2 Answers

The punctured Hopf surface is also an example for the revised question. It is easy to see that the Hopf surface $X$ is parallelizable (since it is diffeomorphic to $S^3\times S^1$); hence, $M=X- \{x\}$ admits an immersion in $R^4={\mathbb C}^2$ (Hirsch-Smale theory). Now pull-back the standard (flat) Kahler structure from ${\mathbb C}^2$ to $M$ via the immersion. On the other hand, as noted in this answer mentioned by Michael Albanese in his comment, the standard complex structure on $M$ does not admit a compatible Kahler metric.

• In previous comment there is a link, your answer is duplicate – user21574 May 12 '17 at 19:40
• @HassanJolany: Duplicate to what? – Misha May 12 '17 at 19:41

In this G&T paper there are examples of complex structures on $\Bbb R^4$ withouth compatible Kähler metrics. The point is that in this $(\Bbb R^4, J)$ there are holomorphic elliptic compact curves, so there are no compatible symplectic structures.

On the other hand, any smooth connected oriented noncompact 4-manifold admits both Kähler and non-Kähler complex structures (cf. this paper). I do not know about higher dimensions, regarding your last question.