Semistable Fibrations on Rational Surfaces with at least 6 Singular   Fibers
  This paper has been withdrawn. A completely rewritten new version will be soon submitted. In order to obtain the inequality $6(g-1) \le K_f^2$ some additional conditions must be imposed on the fibration, involving the number on certain exceptional curves. 
Claudia R. Alcantara, Abel Castorena and Alexis G. Zamora