Singular Fibers and Kodaira Dimensions

Abstract

Let $f:\,X \to \mathbb{P}^1$ be a non-isotrivial semi-stable family of varieties of dimension $m$ over $\mathbb{P}^1$ with $s$ singular fibers. Assume that the smooth fibers $F$ are minimal, i.e., their canonical line bundles are semiample. Then $\kappa(X)\leq \kappa(F)+1$. If $\kappa(X)=\kappa(F)+1$, then $s>\frac{4}m+2$. If $\kappa(X)\geq 0$, then $s\geq\frac{4}m+2$. In particular, if $m=1$, $s=6$ and $\kappa(X)=0$, then the family $f$ is Teichm\"uller.