On threefolds with ${\mathbf K^3=2p_g-6}$

Abstract

It is known that if $X$ is an $n$--dimensional normal variety, and $D$ a nef and big Cartier divisor on it such that the associated map $\varphi_D$ is generically finite then $D^n\geq 2(h^0(X,\Oc_X(D))-n)$. We study the case in which the equality holds for $n=3$ and $D=K_X$ is the canonical divisor. \par We also produce a bound for the admissible degree of the canonical map of a threefold, when it is supposed to be generically finite.