Factoriality of certain threefolds complete intersection in ${bold P^5}$ with ordinary double points.

Let $V\subset \bold P^5$ be a reduced and irreducible threefold of degree $s$, complete intersection on a smooth hypersurface of degree $t$, with $s>t^2-t$. In this paper we prove that if the singular locus of $V$ consists of $\delta < 3s/8t$ ordinary double points, then any projective surface contained in $V$ is a complete intersection on $V$. In particular $V$ is ${\bold Q}$-factorial