ABSTRACT. We study the deformation theory of lines on the Fermat quintic threefold. We formulate an infinitesimal version of the generalized Hodge conjecture, and use our analysis of lines to prove it in a special case.

ABSTRACT. We study the deformation theory of lines on the Fermat quintic threefold. We formulate an infinitesimal version of the generalized Hodge conjecture, and use our analysis of lines to prove it in a special case.

We construct three new families of fibrations π : S → B where S is an algebraic complex surface and B a curve that violate Xiao's conjecture relating the relative irregularity and the genus of the general fiber. The fibers of π are certain étale cyclic covers of hyperelliptic curves that give coverings of P 1 with dihedral monodromy. As an application, we also show the existence of big and nef effective divisors in the Brill-Noether range.

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