The H_N filtration of bundles as Frobenius pull-back
  Let X be a smooth projective curve over an algebraic closed field of characteristic p and F be the Frobenius morphism of X. Here, I give a negative answer to the guess that the length of the Harder-Narasimhan of F*W is not bigger than p, where W is a semistable bundle on X. 
Mingshuo Zhou
math.AG