A new bound on the number of special fibers in a pencil of curves

Abstract

In a paper by J. V. Pereira and the author it was proved that any pencil of plane curves of degree d > 1 d>1 with irreducible generic fiber can have at most five completely reducible fibers although no examples with five such fibers had ever been found. Recently Janis Stipins has proved that if a pencil has a base of d 2 d^2 points, then it cannot have five completely reducible fibers. In this paper we generalize Stipins’ result to arbitrary pencils. We also include into consideration more general special fibers that are the unions of lines and non-reduced curves. These fibers are important for characteristic varieties of hyperplane complements.