Postulation of generic lines and one double line in $\PP^n$ in view of generic lines and one multiple linear space

Abstract

A well-known theorem by Hartshorne--Hirschowitz (\cite{HH}) states that a generic union $\mathbb{X}\subset \PP^n$, $n\geq 3$, of lines has good postulation with respect to the linear system $|\OO_{\PP^n}(d)|$. So a question that arises naturally in studying the postulation of non-reduced positive dimensional schemes supported on linear spaces is the question whether adding a $m$-multiple linear space $m\PP^r$ to $\mathbb{X}$ can still preserve it's good postulation, which means in classical language that, whether $m\PP^r$ imposes independent conditions on the linear system $|\II_{\mathbb{X}}(d)|$. Recently, the case of $r=0$, i.e., the case of lines and one $m$-multiple point, has been completely solved by several authors (\cite{CCG4}, \cite{AB}, \cite{B1}) starting with Carlini--Catalisano--Geramita, while the case of $r>0$ was remained unsolved, and this is what we wish to investigate in this paper. Precisely, we study the postulation of a generic union of $s$ lines and one $m$-multiple linear space $m\PP^r$ in $\PP^n$, $n\geq r+2$. Our main purpose is to provide a complete answer to the question in the case of lines and one double line, which says that the double line imposes independent conditions on $|\II_{\mathbb{X}}(d)|$ except for the only case $\{n=4, s=2, d=2\}$. Moreover, we discuss an approach to the general case of lines and one $m$-multiple linear space, $(m\geq 2, r\geq 1)$, particularly, we find several exceptional such schemes, and we conjecture that these are the only exceptional ones in this family. Finally, we give some partial results in support of our conjecture.