# Rational map to a non-uniruled manifold

Let X, Y be smooth projective (over IC), let f:X..>Y be a rational map. Assume Y is not uniruled. Is it true that f will be regular over a non-empty open subset of Y? Funny..

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Dear Ivor: it might be useful to give some motivation for the question. For instance, do you have a reason to think it is true? –  Artie Prendergast-Smith Mar 31 '11 at 12:04
If $f\colon X \to Y$ is dominant, then there are always non-empty, open subsets $U \subset X$ and $V \subset Y$ such that the restriction $f \colon U \to V$ is a regular map. Why are you requiring $Y$ non-uniruled? –  Francesco Polizzi Mar 31 '11 at 12:50
Dear Francesco, I think the point of the question (which was not immediately clear to me) is that f should have some proper fibres. –  Artie Prendergast-Smith Mar 31 '11 at 13:03
@Francesco: ivor probably means that there exists an open set $V\subset Y$ such that $f$ is regular (i.e., defined) on $f^{-1}(V)$. –  Sándor Kovács Mar 31 '11 at 14:49
@Sandor: Right, that's what I meant (but expressed badly). In his answer he clarifies that this is indeed what is meant. –  Artie Prendergast-Smith Mar 31 '11 at 15:52

I think you have already answered this yourself, for the most part. We have a birational morphism $X' \to X$ with some exceptional locus $E$ inside $X'$, which is some finite union of ruled components $\mathbb{P}^1 \times Z_i$. We also have the (proper) map $f': X' \to Y$, and the restriction of the map to any $\mathbb{P}^1 \times Z_i \to Y$ does not factor through $\mathbb{P}^1 \times Z_i \to Z_i$, so the image inside $Y$ will be uniruled. As such, the image of $E$ inside $Y$ is some closed set $V$ which by hypothesis cannot be all of $Y$. Restricting to the complement $U \subset Y$, the map $f'^{-1}(U) \to U$ has domain away from the exceptional locus inside $X'$, which is what you wanted, yes?

Also, the analysis you gave in your answer almost works, I think, except there is a slight problem in that you can only take $p$ to be very general (in the complement of a countable union of closed proper subsets) rather than merely general. For example, a very general K3 surface has infinitely many rational curves despite not being uniruled, so the locus in $Y$ to which your analysis applies would be precisely the complement of these infinitely many rational curves.

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Thank you Arnav, I think that settles it. –  ivor Apr 1 '11 at 6:54