In algebraic geometry, the secant variety , or the variety of chords, of a projective variety is the Zariski closure of the union of all secant lines (chords) to V in :[1]

(for , the line is the tangent line.) It is also the image under the projection of the closure Z of the incidence variety

.

Note that Z has dimension and so has dimension at most .

More generally, the secant variety is the Zariski closure of the union of the linear spaces spanned by collections of k+1 points on . It may be denoted by . The above secant variety is the first secant variety. Unless , it is always singular along , but may have other singular points.

If has dimension d, the dimension of is at most . A useful tool for computing the dimension of a secant variety is Terracini's lemma.

Examples

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A secant variety can be used to show the fact that a smooth projective curve can be embedded into the projective 3-space   as follows.[2] Let   be a smooth curve. Since the dimension of the secant variety S to C has dimension at most 3, if  , then there is a point p on   that is not on S and so we have the projection   from p to a hyperplane H, which gives the embedding  . Now repeat.

If   is a surface that does not lie in a hyperplane and if  , then S is a Veronese surface.[3]

References

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  • Eisenbud, David; Joe, Harris (2016), 3264 and All That: A Second Course in Algebraic Geometry, C. U.P., ISBN 978-1107602724
  • Griffiths, P.; Harris, J. (1994). Principles of Algebraic Geometry. Wiley Classics Library. Wiley Interscience. p. 617. ISBN 0-471-05059-8.
  • Joe Harris, Algebraic Geometry, A First Course, (1992) Springer-Verlag, New York. ISBN 0-387-97716-3