Positivity of Chern classes for reflexive sheaves on PN

It is well known that the Chern classes ci of a rank n vector bundle on PN, generated by global sections, are non-negative if i ≤ n and vanish otherwise. This paper deals with the following question: does the above result hold for the wider class of reflexive sheaves? We show that the Chern numbers ci with i ≥ 4 can be arbitrarily negative for reflexive sheaves of any rank; on the contrary for i ≤ 3 we show positivity of the ci with weaker hypothesis. We obtain lower bounds for c1, c2 and c3 for every reflexive sheaf \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {F}}$$\end{document} which is generated by \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${H^0\mathcal {F}}$$\end{document} on some non-empty open subset and completely classify sheaves for which either of them reach the minimum allowed, or some value close to it.