\def\beq{\begin{equation}} \def\eeq{\end{equation}} \def\ba{\begin{eqnarray}} \def\ea{\end{eqnarray}} \def\bal{\begin{align}} \def\eal{\end{align}} \def\bxi{{\mbox{\boldmath $\xi$}}} \def\bnab{{\mbox{\boldmath $\nabla$}}} \def\calP{{\cal P}} %\usepackage{multicol} \documentclass{article} \usepackage{amsfonts,amsmath,amssymb} \usepackage{graphicx} \usepackage{color} \usepackage{url} \bibliographystyle{plain}