\newlength{\longarc}
\newcommand{\arc}[1]{\settowidth{\longarc}{$#1$}%
  \addtolength{\longarc}{-0.5em}%
  \unitlength \longarc%
  \ensuremath{%
    \stackrel{\begin{picture}(1,0.2)
    	\qbezier(0,0)(0.5,0.2)(1,0)
	      \end{picture}}
    {#1}
  }}

\usepackage{pstricks}
\newcommand{\arcoriente}[1]{\settowidth{\longarc}{$#1$}%
  \addtolength{\longarc}{-0.5em}%
  \ensuremath{%
    \unitlength \longarc%
    \stackrel{\begin{picture}(1,0.1)
    	\psset{unit=\longarc}
	\pscurve[linewidth=0.5pt]{->}(0,0)(0.5,0.1)(1,0)
    \end{picture}}
    {#1}
  }}
