# When is a holomorphic tangent bundle stable?

Hello, I was thinking about the stability condition in terms of Mumford and I have a question:

If $S$ is a compact Kahler surface (complex 2D), when is the holomorphic tangent bundle $T^{1,0}S$ stable?

When $S$ is Kahler-Einstein, $T^{1,0} S$ is stable by the Donaldson-Uhlenbeck-Yau theorem. But Kahler-Einstein examples are rather restrictive as it requires the hermitian metric $h$ on $TS$ to be induced by the riemannian metric $g$. Would there be more examples other than Kahler-Einstein with stable holomorphic tangent bundles?

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The following has been added to clarify some points.

Appendix 1: What I meant by Kahler-Einstein being "restrictive".

Hermitian-Yang-Mills connections or equivalently Hermite-Einstein metrics can be thought as a generalization of Kahler-Einstein metrics. $T^{1,0}S$ admits a Hermite-Einstein metric with the hermitian structure induced from $(S, J, g)$ when $S$ is a Kahler-Einstein. However, in principle $T^{1,0}S$ can admit a Hermite-Einstein metric even when $S$ is not Kahler-Einstein. In this case, the hermitian structure on $T^{1,0}S$ will be different from the one naturally induced from $(S, J, g)$.

Appendix 2: What I meant by stable bundle.

The stability condition introduced by Mumford is the following.

A vector bundle $V$ is stable if for all coherent sub-sheaves $U$ $$\frac{\deg (U)}{\mathrm{rank} (U)}<\frac{\deg (V)}{\mathrm{rank} (V)}.$$
Here the degree is computed using the Kahler form $\omega$ (polarization). By the Donaldson-Uhlenbeck-Yau theorem, on Kahler manifolds stability is equivalent to $V$ admitting a Hermitian-Yang-Mills connection. This theorem was later generalized by Li and Yau assuming stability to non-Kahler manifolds.

• Maybe you can use the Yang-Mills interpretation of a Hermitian-Einstein connection.
– Acky
Mar 7, 2013 at 13:21

What do you mean by stable? usually stable is with respect to a polarization $$H$$. If $$K_X$$ is ample and you choose $$H=K_X$$ then we know by Aubin-Yau's theorem that there exists a Kahler-Einstein metric. Thus, in this case, the existence of such a metric is not restrictive.
If you choose any ample line bundle $$H$$, then Donaldson has proved that if a holomorphic vector bundle $$E$$ over a compact Kahler manifold $$M$$ admits an approximate $$\omega$$-Einstein-Hermitian structure if and only if $$E$$ is $$H$$-semistable, where $$\omega$$ is a Kahler form in the class of $$H$$.
• Hello, thanks for the reply. Perhaps I should try to explain what I meant by Kahler-Einstein metrics being "restrictive". Hermitian-Yang-Mills connection (also known as Hermite-Einstein metrics) can be thought of a generalization of Kahler-Einstein metrics with Levi-Civita connection. In that sense, Kahler-Einstein metrics are trivial. Note that $T^{1,0}S$ can admit an Hermite-Einstein metric without $S$ being Kahler-Einstein. This can be achieved by using a different hermitian structure on $T^{1,0}S$ rather than the natural one coming from $(S,J,g,\omega)$. Oct 6, 2011 at 8:58
• Every line bundle $L$, on Kahler variety $X$ is stable in general