Projections from Subvarieties

Abstract

Let $X\subset P^N$ be an n-dimensional connected projective submanifold of projective space. Let $p:P^N\to P^{N-q-1}$ denote the projection from a linear $P^q\subset P^N$. Assuming that $X\not\subset P^q$ we have the induced rational mapping $\psi :=p_X:X\to P^{N-q-1}$. This article started as an attempt to understand the structure of this mapping when $\psi$ has a lower dimensional image. In this case of necessity we have $Y:=X\cap P^q$ is nonempty. We have in this article studied a closely related question, which includes many special cases including the case when the center of the projection $\pn q$ is contained in $X$. PROBLEM. Let $Y$ be a proper connected k-dimensional projective submanifold of an $n$-dimensional projective manifold $X$. Assume that $k>0$. Let $L$ be a very ample line bundle on $X$ such that $L\otimes I_Y$ is spanned by global sections, where $I_Y$ denotes the ideal sheaf of $Y$ in $X$. Describe the structure of $(X,Y,L)$ under the additional assumption that the image of $X$ under the mapping $\psi$ associated to $|L\otimes I_Y|$ is lower dimensional.