Secant variety

In algebraic geometry, the secant variety $\mathrm{Sect}(V)$, or the variety of chords, of a projective variety $V\subset {\mathbb{P}}^r$ is the Zariski closure of the union of all secant lines (chords) to V in ${\mathbb{P}}^r$:

$\mathrm{Sect}(V)=\bigcup _{x,y\in V}\overline{xy}$

(for $x=y$, the line $\overline{xy}$ is the tangent line.) It is also the image under the projection $p_3:({\mathbb{P}}^r{)}^3\to {\mathbb{P}}^r$ of the closure Z of the incidence variety

$\{(x,y,l)|x\wedge y\wedge l=0\}$.

Note that Z has dimension $2\dim V+1$ and so $\mathrm{Sect}(V)$ has dimension at most $2\dim V+1$.

More generally, the $k^{th}$ secant variety is the Zariski closure of the union of the linear spaces spanned by collections of k+1 points on $V$. It may be denoted by ${\mathrm{\Sigma }}_k$. The above secant variety is the first secant variety. Unless ${\mathrm{\Sigma }}_k={\mathbb{P}}^r$, it is always singular along ${\mathrm{\Sigma }}_{k-1}$, but may have other singular points.

If $V$ has dimension d, the dimension of ${\mathrm{\Sigma }}_k$ is at most $kd+d+k$. A useful tool for computing the dimension of a secant variety is Terracini's lemma.

Examples

A secant variety can be used to show the fact that a smooth projective curve can be embedded into the projective 3-space ${\mathbb{P}}^3$ as follows. Let $C\subset {\mathbb{P}}^r$ be a smooth curve. Since the dimension of the secant variety S to C has dimension at most 3, if $r>3$, then there is a point p on ${\mathbb{P}}^r$ that is not on S and so we have the projection ${\pi }_p$ from p to a hyperplane H, which gives the embedding ${\pi }_p:C\hookrightarrow H\simeq {\mathbb{P}}^{r-1}$. Now repeat.

If $S\subset {\mathbb{P}}^5$ is a surface that does not lie in a hyperplane and if $\mathrm{Sect}(S)\ne {\mathbb{P}}^5$, then S is a Veronese surface.

References

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• Joe Harris, Algebraic Geometry, A First Course, (1992) Springer-Verlag, New York. Template:Isbn

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