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

Abstract

In the previous paper by Pereira and the author, it was proved that any pencil of plane curves of degree greater than one with irreducible generic fiber can have at most five completely reducible fibers although no examples with five such fibers were ever found. Recently Janis Stipins has proved that if any two fibers of a pencil intersect transversally then it cannot have five completely reducible fibers. In this paper we generalize the 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 line complements.