BLOWUPS AND FIBERS OF MORPHISMS

Abstract

Our object of study is a rational map  defined by homogeneous forms $g_{1},\ldots ,g_{n}$, of the same degree $d$, in the homogeneous coordinate ring $R=k[x_{1},\ldots ,x_{s}]$ of $\mathbb{P}_{k}^{s-1}$. Our goal is to relate properties of $\unicode[STIX]{x1D6F9}$, of the homogeneous coordinate ring $A=k[g_{1},\ldots ,g_{n}]$ of the variety parameterized by $\unicode[STIX]{x1D6F9}$, and of the Rees algebra ${\mathcal{R}}(I)$, the bihomogeneous coordinate ring of the graph of $\unicode[STIX]{x1D6F9}$. For a regular map $\unicode[STIX]{x1D6F9}$, for instance, we prove that ${\mathcal{R}}(I)$ satisfies Serre’s condition $R_{i}$, for some $i>0$, if and only if $A$ satisfies $R_{i-1}$ and $\unicode[STIX]{x1D6F9}$ is birational onto its image. Thus, in particular, $\unicode[STIX]{x1D6F9}$ is birational onto its image if and only if ${\mathcal{R}}(I)$ satisfies $R_{1}$. Either condition has implications for the shape of the core, namely, $\text{core}(I)$ is the multiplier ideal of $I^{s}$ and $\text{core}(I)=(x_{1},\ldots ,x_{s})^{sd-s+1}.$ Conversely, for $s=2$, either equality for the core implies birationality. In addition, by means of the generalized rows of the syzygy matrix of $g_{1},\ldots ,g_{n}$, we give an explicit method to reduce the nonbirational case to the birational one when $s=2$.