On threefolds with the smallest nontrivial monodromy group

Abstract

Using an adjunction-theoretic result due to A.J.Sommese together with a proposition from SGA7, we obtain a complete list of smooth threefolds for which the monodromy group acting on $H^2$ of its smooth hyperplane section is $\mathbb{Z}/2\mathbb{Z}$. The possibility of such a classification was announced by F.L.Zak in 1991.

arXiv comment22 pages, Section 5.3 substantially revised