In this paper we prove that a nodal hypersurface in P^4 with defect has at
least (d-1)^2 nodes, and if it has at most 2(d-2)(d-1) nodes and d>6 then it
contains either a plane or a quadric surface. Furthermore, we prove that a
nodal double cover of P^3 ramified along a surface of degree 2d with defect has
at least d(2d-1) nodes. We construct the largest dimensional family of nodal
degree d hypersurfaces in P^(2n+2) with defect for d sufficiently large.Comment: v2: A proof for the Ciliberto-Di Gennaro conjecture is added (Section
5); Some minor corrections in the other sections. v3: some minor corrections
in the abstract v4: The proof for the Ciliberto-Di Gennaro conjecture has
been modified; The paper is split into two parts, the complete intersection
case will be discussed in a different pape