On varieties of almost minimal degree in small codimension

Abstract

The aim of the present exposition is to investigate varieties of almost minimal degree and of low codimension, in particular their Betti diagrams. Here minimal degree is defined as $\deg X = \codim X + 2.$ We describe the structure of the minimal free resolution of a variety $X$ of almost minimal degree of $\codim X \leq 4$ by listing the possible Betti diagrams. The most surprising fact is, that the non-arithmetically Cohen-Macaulay case of varieties of almost minimal degree can occur only in small dimensions (cf. Section 2 for the precise statements). Our main technical tool is a result shown by the authors (cf. \cite{BS}), which says that besides of an exceptional case, (that is the generic projection of the Veronese surface in $\mathbb P^5_K$) any non-arithmetically normal (and in particular non-arithmetically Cohen-Macaulay) variety of almost minimal degree $X \subset \mathbb P^r_K$ (which is not a cone) is contained in a variety of minimal degree $Y \subset \mathbb P^r_K$ such that $\codim(X,Y) = 1.