On the ideals of Secant Varieties to certain rational varieties
  If $\X \subset \P^n$ is a reduced and irreducible projective variety, it is interesting to find the equations describing the (higher) secant varieties of $\X$. In this paper we find those equations in the following cases:   $\X = \P^{n_1}\times...\times\P^{n_t}\times\P^n$ is the Segre embedding of the product and $n$ is "large" with respect to the $n_i$ (Theorem 2.4);   $\X$ is a Segre-Veronese embedding of some products with 2 or three factors;   $\X$ is a Del Pezzo surface. 
M.V. Catalisano, A.V. Geramita, A. Gimigliano