On Secant Loci and Simple Linear Projections of Some Projective Varieties

In this paper, we study how simple linear projections of some projective varieties behave when the projection center runs through the ambient space. More precisely, let X ⊂ P be a projective variety satisfying Green-Lazarsfeld’s property Np for some p ≥ 2, q ∈ P a closed point outside of X , and Xq := πq(X) ⊂ P the projected image of X from q. First, it is shown that the secant locus Σq(X) of X with respect to q, i.e. the set of all points onX spanning secant lines passing through q, is either empty or a quadric in a subspace of P. This implies that the finite morphism πq : X → Xq is birational. Our main result is that cohomological and local properties of Xq are precisely determined by Σq(X). To complete this result, the next step should be to classify all possible secant loci and to decompose the ambient space via the classification of secant loci. We obtain such a decomposition for Veronese embeddings and Segre embeddings. Also as an application of the main result, we study cohomological properties of low degree varieties.