(A) study on the variation of second fundamental forms on the intersection of two quadric hypersurfaces

Abstract

In [GH], Griffiths and Harris asked whether a projective complex submanifold of codimension two is determined by the moduli of its second fundamental forms. More precisely, given a nonsingular subvariety $X^n$ ⊂ $P^{n+2}$, the second fundamental form $II_{X,x}$ at a point x ∈ X is a pencil of quadrics on $T_x(X)$, defining a rational map $µ^x$ from X to a suitable moduli space of pencils of quadrics on a complex vector space of dimension n. The question raised by Griffiths and Harris was whether the image of $µ^X$ determines X. We study this question when $X^n$ ⊂ $P^{n+2}$ is a nonsingular intersection of two quadric hypersurfaces of dimension n > 4. In this case, the second fundamental form $II_{X,x}$ at a general point x ∈ X is a nonsingular pencil of quadrics. Firstly, we prove that the moduli map $µ^X$ is dominant over the moduli of nonsingular pencils of quadrics. This gives a negative answer to Griffiths-Harris’s question. To remedy the situation, we consider a refined version $µe^X$ of the moduli map $µ^X$, which takes into account the infinitesimal information of $\widetilde\mu^X$. Our main result is an affirmative answer in terms of the refined moduli map: we prove that the image of $\widetilde\mu^X$ determines X, among nonsingular intersections of two quadrics.