Scorza variety

In mathematics, a k-Scorza variety is a smooth projective variety, of maximal dimension among those whose k–1 secant varieties are not the whole of projective space. Scorza varieties were introduced and classified by Zak (1993), who named them after Gaetano Scorza. The special case of 2-Scorza varieties are sometimes called Severi varieties, after Francesco Severi.


Zak showed that k-Scorza varieties are the projective varieties of the rank 1 matrices of rank k simple Jordan algebras.

Severi varieties

The Severi varieties are the non-singular varieties of dimension n (even) in PN that can be isomorphically projected to a hyperplane and satisfy N=3n/2+2.

These 4 Severi varieties can be constructed in a uniform way, as orbits of groups acting on the complexifications of the 3 by 3 hermitian matrices over the four real (possibly non-associative) division algebras of dimensions 2k = 1, 2, 4, 8. These representations have complex dimensions 3(2k+1) = 6, 9, 15, and 27, giving varieties of dimension 2k+1 = 2, 4, 8, 16 in projective spaces of dimensions 3(2k)+2 = 5, 8, 14, and 26.

Zak proved that the only Severi varieties are the 4 listed above, of dimensions 2, 4, 8, 16.


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