On the Universal Scheme of $r$-relative clusters of a family

Abstract

We generalize the concept of $r$-point clusters of a scheme $S$ to $r$-relative clusters of a $B$-scheme $\mathcal{S}$. Define schemes $C{l}_r$ that naturally parametrize the $r$-relative clusters which generalize the Kleiman's construction of the iterated blowups by $r$-point clusters. We show that the iterated construction of $C{l}_{r+1}$ from $C{l}_r$ is something more than just to do a blow up of $C{l}_r{\times }_{C{l}_{r-1}}C{l}_r$ at the diagonal. We show a few simple examples illustrating the situations that may occur.

arXiv comment20 pages, fixed wrong lemma, results unchanged

References in corpus (1)

• Threefolds of P^5 with one apparent quadruple point