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4 papers
Trisecant lemma for nonequidimensional varieties
J. Y. Kaminskia,
A. Ya. Kanel-Belovb,
M. Teicherc a Holon Academic Institute of Technology
b Hebrew University of Jerusalem
c Bar-Ilan University, Department of Chemistry
Abstract:
Let
$X$ be an irreducible projective variety over an algebraically closed field of characteristic zero. For
$r \geq3$, if every
$(r-2)$-plane
$\overline{x_1,\dots,x_{r-1}}$, where the
$x_i$ are generic points, also meets
$X$ in a point
$x_r$ different from
$x_1,\dots,x_{r-1}$, then
$X$ is contained in a linear subspace
$L$ such that
$\operatorname{codim}_L X \leq r-2$. In this paper, our purpose is to present another derivation of this result for
$r=3$ and then to introduce a generalization to nonequidimensional varieties. For the sake of clarity, we shall reformulate our problem as follows. Let
$Z$ be an equidimensional variety (maybe singular and/or reducible) of dimension
$n$, other than a linear space, embedded into
$\mathbb P^r$, where
$r \geq n+1$. The variety of trisecant lines of
$Z$, say
$V_{1,3}(Z)$, has dimension strictly less than
$2n$, unless
$Z$ is included in an
$(n+1)$-dimensional linear space and has degree at least 3, in which case
$\dim V_{1,3}(Z) = 2n$. This also implies that if
$\dim V_{1,3}(Z)=2n$, then
$Z$ can be embedded in
$\mathbb P^{n+1}$. Then we inquire the more general case, where
$Z$ is not required to be equidimensional. In that case, let
$Z$ be a possibly singular variety of dimension
$n$, which may be neither irreducible nor equidimensional, embedded into
$\mathbb P^r$, where
$r\geq n+1$, and let
$Y$ be a proper subvariety of dimension
$k\geq1$. Consider now
$S$ being a component of maximal dimension of the closure of $\{l \in\mathbb G(1,r)\mid\exists p\in Y,\ q_1,q_2\in Z\setminus Y,q_1,q_2,p\in l\}$. We show that
$S$ has dimension strictly less than
$n+k$, unless the union of lines in
$S$ has dimension
$n+1$, in which case
$\dim S=n+k$. In the latter case, if the dimension of the space is strictly greater than
$n+1$, then the union of lines in
$S$ cannot cover the whole space. This is the main result of our paper. We also introduce some examples showing that our bound is strict.
UDC:
512.7