Skip to main content
2 of 3
deleted 542 characters in body
Francesco Polizzi
  • 66.3k
  • 5
  • 180
  • 283

Smooth, non-isotrivial fibration with vanishing Kodaira-Spencer map at a point

This question arose by reading the paper [1], in particular, the remark at p. 737:

As an example, consider a non-isotrivial smooth projective morphism $f \colon X \to Y$ from a smooth projective surface to a smooth projective curve. Suppose that there exists a $y \in Y$ such that the Kodaira-Spencer map $$\rho_{f, \, y} \colon T_{Y, \, y} \to H^1(X_y, \, T_{X_y})$$ is zero. Then $\Omega_X|_{X_y}=\mathcal{O}_{X_y} \oplus \Omega_{X_y}$, so $\Omega_X|_{X_y}$ is not ample.

Smooth, non-isotrivial fibrations from a surface to a curve are called Kodaira fibrations (see [2] for a good introduction to the subject) and the usual way to construct them is taking suitable ramified covers of a product of two curves.

Now, I do not see how to perform the construction in such a way that the above condition on the Kodaira-Spencer map is satisfied. So let me ask the

Question. What are examples of non-isotrivial smooth projective morphisms $f \colon X \to Y$ from a smooth projective surface to a smooth projective curve such that the Kodaira-Spencer map vanishes at some $y \in Y$?

References.

[1] Jabbusch, Kelly, Positivity of cotangent bundles, Mich. Math. J. 58, No. 3, 723-744 (2009). ZBL1186.14037.

[2] Catanese, Fabrizio, Kodaira fibrations and beyond: methods for moduli theory, Jpn. J. Math. (3) 12, No. 2, 91-174 (2017). ZBL1410.14010.

Francesco Polizzi
  • 66.3k
  • 5
  • 180
  • 283